[SYCL] Fix zstd build when static zstd libraries are not found

[uditagarwal97]

Problem:

When we use LLVM_ENABLE_ZSTD=ON along with LLVM_USE_STATIC_ZSTD=ON, LLVM config fails with the following error when libzstd.a is not found but libzstd.so is found:

CMake Error at lib/Support/CMakeLists.txt:327 (get_property):
  get_property could not find TARGET zstd::libzstd_static.  Perhaps it has
  not yet been created.


CMake Error at lib/Support/CMakeLists.txt:330 (get_property):
  get_property could not find TARGET zstd::libzstd_static.  Perhaps it has
  not yet been created.

The bug is in upstream and in the following lines: https://github.com/intel/llvm/blob/sycl/llvm/lib/Support/CMakeLists.txt#L32 - In the else(), the code just assumes that static libraries are available.

Solution:

When user specifies LLVM_USE_STATIC_ZSTD=ON, we should enable ZSTD only if static zstd library is present. Otherwise, the build should just proceed normally since zstd is optional, even in upstream.

[uditagarwal97]

I'll create a bug report and PR to upstream this change, if approved.

[uditagarwal97]

For upstreaming:
issue: llvm/llvm-project#113583
PR: llvm/llvm-project#113584

[sarnex]

sorry what is zstd::libzstd_static and who sets it? is it set by find_package if it finds the static lib? not a cmake expert

[uditagarwal97]

Yes, zstd::libzstd_static is set during find_package if it finds static ZSTD libraries. https://github.com/intel/llvm/blob/sycl/llvm/cmake/modules/Findzstd.cmake#L60

[sarnex]

makes sense to me, but lets also submit to upstream like you said

[uditagarwal97]

@intel/llvm-gatekeepers the PR is ready to be merged.