[cmake] avoid use of undocumented FindThread module variable

[carenas]

Summary

df7103ddc555 ([build] Make sure to link main executable with pthreads,
2016-06-21) add a reference to an internal variable that was meant to
ensure the returned flag would be a library name.

Change the variable name to the documented[1] one and that was made
available since release of cmake 3.1.

After this change; switching its value to "true" and removing the TODO
should be possible when using newer cmake, and indeed seems to work
fine under limited testing, but that change has been punted until a
need arises (ex: RISCV with gcc failing to link because of missing
libatomic) since it could also be problematic (ex: older clang warning
when it is used while linking)

[1] https://cmake.org/cmake/help/latest/module/FindThreads.html#variable:THREADS_PREFER_PTHREAD_FLAG

[beanz]

Looks good to me. Just to leave a paper trail, this is also documented in the CMake 3.13 documentation (which is our current minimum required version):
https://cmake.org/cmake/help/v3.13/module/FindThreads.html

[carenas]

Could someone merge this change for me (I have no merge rights)?, build errors in the precheck are unrelated, but more importantly have failed in different places on each run so they are not indicative of an issue that would block this AFAIK

llvm/cmake/config-ix.cmake

 # Check for libpfm.
 include(FindLibpfm)
 
 if(HAVE_LIBPTHREAD)
   # We want to find pthreads library and at the moment we do want to
   # have it reported as '-l<lib>' instead of '-pthread'.
   # TODO: switch to -pthread once the rest of the build system can deal with it.
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
-  set(THREADS_HAVE_PTHREAD_ARG Off)
+  set(THREADS_PREFER_PTHREAD_FLAG FALSE)
   find_package(Threads REQUIRED)
   set(LLVM_PTHREAD_LIB ${CMAKE_THREAD_LIBS_INIT})
 endif()
 
 if(LLVM_ENABLE_ZLIB)
   if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON)
     find_package(ZLIB REQUIRED)
   elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")