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!

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@206850 91177308-0d34-0410-b5e6-96231b3b80d8