Change llvm-config --ldflags to report ${CMAKE_CXX_LINK_FLAGS}

Should fix PR19526.

When Oscar added this code in the intial CMake build system port, he had
a TODO saying that ${CMAKE_SHARED_LINKER_FLAGS} was probably wrong.  I
agree.  I'm using ${CMAKE_CXX_LINK_FLAGS} to point LLVM at my custom
installation of gcc 4.recent, so that seems more correct.  With this
change, I can build creduce against an installed clang, and it picks up
the write flags from --ldflags.