build: use keyword signatures for link dependencies

This updates the target_link_libraries invocation when building with CMake
2.8.12 or newer.  The newer version of CMake warns when keyword and plain
signatures are used for target_link_libraries.  This addresses that difference
while maintaining compatibility with the older releases by defining a macro for
the keyword that is defined based on the current version of CMake.

Patch originally by chapuni!

llvm-svn: 206850