[libclc] [libclc] Fix Unix Makefiles race condition in add_libclc_builtin_library (PR #187449)

[Wenju He via cfe-commits]

wenju-he wrote:

this race condition exposed a limitation in runtimes configuration: main runtime-build can be built in parallel with a specific libclc (in EXTRA_TARGETS) when a non-runtime target depends on libclc.
Improve it in https://github.com/llvm/llvm-project/pull/187634

https://github.com/llvm/llvm-project/pull/187449