Use LLVMLibsOptions intead of LDFLAGS when setting the libclang soname. This allows 'make LDFLAGS=foo' to be set correctly (and matches the way it is done with libllvm and liblldb)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193607 91177308-0d34-0410-b5e6-96231b3b80d8