Prevent LLVM cmake from finding builtin zstd.

[pcanal]

• Fix removal of ROOTSYS from CMAKE_PREFIX_PATH.
  Current code was, if CMAKE_PREFIX_PATH had more than one element, essentially
  inadvertently replacing ROOTSYS by the current path. (eg. if CMAKE_PREFIX_PATH
  contains just : or :: or :somethingelse, it will add the current directory (top level
  of the build directory to the prefix path).

• Fix find_package disabling for zstd.
  LLVM uses the lower case spelling for 'zstd' and thus find_package was still being called.

In practice this lead my build: -Dbuiltin_zstd=ON -Dbuiltin_xxhash=ON -Dminimal=ON -Dbuiltin_zlib=ON on AlmaLinux release 9.5 (Teal Serval) to pick up the lib/libstd.a for the LLVM libraries and fails to link rootcling_stage1 because of missing xxHash symbols.

This PR also fixes: its.cern.ch/jira/browse/ROOT-10537

This is somewhat related to #16285
and #8633

[ferdymercury]

Somewhat related: https://its.cern.ch/jira/browse/ROOT-10537

[pcanal]

You are right. I will fix that issue as part of this PR.

[github-actions]

Test Results

    18 files      18 suites   4d 6h 25m 7s ⏱️
 2 725 tests  2 721 ✅ 0 💤 4 ❌
47 359 runs  47 355 ✅ 0 💤 4 ❌

For more details on these failures, see this check.

Results for commit 6a97247b.

♻️ This comment has been updated with latest results.

[pcanal]

Sigh ... But sometimes we seem to want to use the builtin libzstd:

  input_line_1:1:2: fatal error: malformed or corrupted AST file: 'LLVM was not built with LLVM_ENABLE_ZSTD or did not find zstd at build time'
  #include <new>
   ^
  input_line_1:1:2: note: after modifying system headers, please delete the module cache at '/github/home/ROOT-CI/build/lib'

i.e. either libzstd is not installed locally or it is no longer found.

[pcanal]

Sigh ... But sometimes we seem to want to use the builtin libzstd:

Right ... :( ... Actually the error message is triggered by reading the previously produced pcm files. I.e. since LLVM could not use the builtin and further did not actually run any find_package in that build (both zlib and zstd are built-in) there is no compression done on the pcm ... so it is a bit too bad that we can't have the builtins used by LLVM.
(and the builds are fixed by deleting the existing pcm files)

[hageboeck]

The cleanup of the prefix path looks reasonable.

Just one clarification regarding the pcms:

so it is a bit too bad that we can't have the builtins used by LLVM. (and the builds are fixed by deleting the existing pcm files)

When LLVM wasn't configured with compression libraries, this still results in functioning pcms, just uncompressed? In that case I think we can go ahead.

[pcanal]

When LLVM wasn't configured with compression libraries, this still results in functioning pcms, just uncompressed?

That is correct.

[pcanal]

@hageboeck I updated to consistent apply the fix to all 3 variable ... and extend to fix https://its.cern.ch/jira/browse/ROOT-10537

[hageboeck]

I updated to consistent apply the fix to all 3 variable

Great, I was going to ask that! 🙂
I think I found a mistake (the replacements should start from the original ENV, not from the temporary variable, which is always empty.
And using a larger regex, one can probably save a few lines.

[hageboeck]

LGTM!

[pcanal]

The failure is unrelated (something about numba).