[cmake] Update creation of object library dependencies for LINK_LIBS PUBLIC

[stephenneuendorffer]

Summary

We need to avoid declaring dependencies on strings which are valid
LINK_LIBS and not valid targets. Previously, we used if(TARGET) to
check this condition. However, if(TARGET) checks whether a target has
been created (in the cmake subdirectory traversal order) and not
whether it *will* be created. This results in annoying directory
ordering problems.

This patch changes the check to more explicitly eliminate problematic
libraries (namely -lpthread) using a REGEX.

[mehdi_amini]

llvm/cmake/modules/AddLLVM.cmake

487

Probably anything that starts with - isn't intended to be a dependency...

llvm/cmake/modules/AddLLVM.cmake

if((ARG_SHARED AND ARG_STATIC) OR ARG_OBJECT)
     # the generated header is created before this object library.
     if(ARG_LINK_LIBS)
       cmake_parse_arguments(LINK_LIBS_ARG
         ""
         ""
         "PUBLIC;PRIVATE"
         ${ARG_LINK_LIBS})
       foreach(link_lib ${LINK_LIBS_ARG_PUBLIC})
-        if(TARGET ${link_lib})
+        # Can't specify a dependence on -lpthread
+        if(NOT ${link_lib} MATCHES "-l.*")
           add_dependencies(${obj_name} ${link_lib})
         endif()
       endforeach()
     endif()
   endif()
 
   if(ARG_SHARED AND ARG_STATIC)
     # static