[CMake] Do all availability checks with -D_GNU_SOURCE

[tambry]

When compiling LLVM with -std=c++ instead of -std=gnu we'd fail to detect many newer POSIX functions.
We define it for the whole of LLVM anyway so moving the definition to the top fixes detection of a bunch of these on such setups.

On my machine:
CMakeCache_pre.txt
CMakeCache_post.txt

[llvm-ci]

LLVM Buildbot has detected a new failure on builder lld-x86_64-win running on as-worker-93 while building llvm at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/146/builds/1648

Here is the relevant piece of the build log for the reference

Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: Support/./SupportTests.exe/37/87' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe-LLVM-Unit-8900-37-87.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=87 GTEST_SHARD_INDEX=37 C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe
--

Script:
--
C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe --gtest_filter=ProgramEnvTest.CreateProcessLongPath
--
C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp(160): error: Expected equality of these values:
  0
  RC
    Which is: -2

C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp(163): error: fs::remove(Twine(LongPath)): did not return errc::success.
error number: 13
error message: permission denied



C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp:160
Expected equality of these values:
  0
  RC
    Which is: -2

C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp:163
fs::remove(Twine(LongPath)): did not return errc::success.
error number: 13
error message: permission denied




********************

[llvm-ci]

LLVM Buildbot has detected a new failure on builder clang-s390x-linux-multistage running on systemz-1 while building llvm at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/98/builds/666

Here is the relevant piece of the build log for the reference

Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'libFuzzer-s390x-default-Linux :: fuzzer-timeout.test' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang    -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta   --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest
RUN: at line 2: /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang    -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta   --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutEmptyTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutEmptyTest
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutEmptyTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutEmptyTest
RUN: at line 3: not  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1 2>&1 | FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test --check-prefix=TimeoutTest
+ not /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1
+ FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test --check-prefix=TimeoutTest
/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test:7:14: error: TimeoutTest: expected string not found in input
TimeoutTest: #0
             ^
<stdin>:19:44: note: scanning from here
==1431496== ERROR: libFuzzer: timeout after 1 seconds
                                           ^
<stdin>:24:104: note: possible intended match here
AddressSanitizer: CHECK failed: asan_report.cpp:199 "((current_error_.kind)) == ((kErrorKindInvalid))" (0x1, 0x0) (tid=1431496)
                                                                                                       ^

Input file: <stdin>
Check file: /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           .
           .
           .
          14: MS: 4 InsertByte-ShuffleBytes-CrossOver-ChangeByte-; base unit: c74064c97231114b678d2204fde8965f65080db6 
          15: 0x48,0x69,0x21,0x2a, 
          16: Hi!* 
          17: artifact_prefix='./'; Test unit written to ./timeout-ba3a9ae8d38b01be61f40be9b84584b7eea146c6 
          18: Base64: SGkhKg== 
          19: ==1431496== ERROR: libFuzzer: timeout after 1 seconds 
check:7'0                                                X~~~~~~~~~~ error: no match found
          20: AddressSanitizer:DEADLYSIGNAL 
check:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          21: ================================================================= 
check:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          22: AddressSanitizer:DEADLYSIGNAL 
check:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          23: ================================================================= 
check:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          24: AddressSanitizer: CHECK failed: asan_report.cpp:199 "((current_error_.kind)) == ((kErrorKindInvalid))" (0x1, 0x0) (tid=1431496) 
check:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
check:7'1                                                                                                            ?                         possible intended match
...