Make sure to get the resource directory from the right clang compiler

[rherilier]

The resource directory is always retrieved from the system's default clang compiler instead of the targeted one.

I notice it after removing clang from my system (which is an alias for clang-16) while keeping clang-19 installed.

[MaskRay]

I download prebuilt Clang packages on https://releases.llvm.org/download.html then set CMAKE_PREFIX_PATH to the root directory of the extracted tarball. ``Clang_EXECUTABLE` will respect that.

clang-${Clang_VERSION_MAJOR} will not work if the distribution only provides clang but not something like clang-19.

[rherilier]

Thanks, I did not expect any differences between Windows and UNICES builds...