Fix find_package(xsimd) for xtl enabled xsimd

[kou]

If we enable xtl by -DENABLE_XTL_COMPLEX=ON, the xsimd CMake target
depends on the xtl CMake target. (See installed
xsimdTargets.cmake. You can find 'INTERFACE_LINK_LIBRARIES "xtl"' line
in the file.)

But installed xsimdConfig.cmake doesn't include
find_dependency(xtl). If we don't run find_dependency(xtl) in
xsimdConfig.cmake, the xtl CMake target isn't defined when users run
find_package(xsimd). If the xtl CMake target isn't defined, "xtl" in
'INTERFACE_LINK_LIBRARIES "xtl"' is processed as library name not
CMake target. If "xtl" is processed as library name, '-lxtl' is added
to link command line. It causes link error because 'libxtl.so' doesn't
exist.

How to reproduce:

1. Install xsimd with -DENABLE_XTL_COMPLEX=ON:

   cmake -S . -B build -DCMAKE_INSTALL_PREFIX=/tmp/local -DENABLE_XTL_COMPLEX=ON
   cmake --build build --target install
   

2. Create a test project:

   xsimd_test.cc:

   #include <xsimd/xsimd.hpp>
   
   int main(void) {
     return 0;
   }
   

   CMakeLists.txt:

    cmake_minimum_required(VERSION 3.1)
    project(xsimd_test)
   
    # Enable <PackageName_ROOT CMake variable for find_package().
    cmake_policy(SET CMP0074 NEW)
   
    find_package(xsimd REQUIRED)
    add_executable(xsimd_test xsimd_test.cc)
    target_link_libraries(xsimd_test xsimd)
   

3. Build the test project:

    $ cmake -S . -B build -Dxsimd_ROOT=/tmp/local
    -- The C compiler identification is GNU 11.3.0
    -- The CXX compiler identification is GNU 11.3.0
    -- Detecting C compiler ABI info
    -- Detecting C compiler ABI info - done
    -- Check for working C compiler: /bin/cc - skipped
    -- Detecting C compile features
    -- Detecting C compile features - done
    -- Detecting CXX compiler ABI info
    -- Detecting CXX compiler ABI info - done
    -- Check for working CXX compiler: /bin/c++ - skipped
    -- Detecting CXX compile features
    -- Detecting CXX compile features - done
    -- Configuring done
    -- Generating done
    -- Build files have been written to: /tmp/abc/build
    $ cmake --build build --verbose
    /usr/bin/cmake -S/tmp/abc -B/tmp/abc/build --check-build-system CMakeFiles/Makefile.cmake 0
    /usr/bin/cmake -E cmake_progress_start /tmp/abc/build/CMakeFiles /tmp/abc/build//CMakeFiles/progress.marks
    /bin/gmake  -f CMakeFiles/Makefile2 all
    gmake[1]: Entering directory '/tmp/abc/build'
    /bin/gmake  -f CMakeFiles/xsimd_test.dir/build.make CMakeFiles/xsimd_test.dir/depend
    gmake[2]: Entering directory '/tmp/abc/build'
    cd /tmp/abc/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /tmp/abc /tmp/abc /tmp/abc/build /tmp/abc/build /tmp/abc/build/CMakeFiles/xsimd_test.dir/DependInfo.cmake --color=
    gmake[2]: Leaving directory '/tmp/abc/build'
    /bin/gmake  -f CMakeFiles/xsimd_test.dir/build.make CMakeFiles/xsimd_test.dir/build
    gmake[2]: Entering directory '/tmp/abc/build'
    [ 50%] Building CXX object CMakeFiles/xsimd_test.dir/xsimd_test.cc.o
    /bin/c++  -isystem /tmp/local/include  -MD -MT CMakeFiles/xsimd_test.dir/xsimd_test.cc.o -MF CMakeFiles/xsimd_test.dir/xsimd_test.cc.o.d -o CMakeFiles/xsimd_test.dir/xsimd_test.cc.o -c /tmp/abc/xsimd_test.cc
    [100%] Linking CXX executable xsimd_test
    /usr/bin/cmake -E cmake_link_script CMakeFiles/xsimd_test.dir/link.txt --verbose=1
    /bin/c++ -rdynamic CMakeFiles/xsimd_test.dir/xsimd_test.cc.o -o xsimd_test  -lxtl
    /bin/ld: cannot find -lxtl: No such file or directory
    collect2: error: ld returned 1 exit status
    gmake[2]: *** [CMakeFiles/xsimd_test.dir/build.make:97: xsimd_test] Error 1
    gmake[2]: Leaving directory '/tmp/abc/build'
    gmake[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/xsimd_test.dir/all] Error 2
    gmake[1]: Leaving directory '/tmp/abc/build'
    gmake: *** [Makefile:91: all] Error 2
   

[JohanMabille]

thanks!