Continue the work started by Dragos' Makefile patch (LLVMLibsOptions usage)

Also replace hard-coded paths with a variable.
Fixed a comment.
Included missing dependencies on Mac OS X (lldbPluginDynamicLoaderPOSIX.a
is a common dependency).
We can't use EXPORTED_SYMBOL_FILE on Darwin right now, don't pass it to
the linker just yet.

llvm-svn: 157618