Link LLVM libraries into libLLVMPolly if BUILD_SHARED_LIBS=ON is set

Without this change we get linker errors such as:

  undefined reference to `llvm::dbgs()'

We only conditionally link in these libraries, as in BUILD_SHARED_LIBS=OFF mode,
linking in these libraries causes such functions (and especially global options)
to be defined twice. The "solution" I choose is most likely not ideal, but seems
to work. If any cmake specialist can suggest a better approach, this would be
appreciated.

We also drop a .c file that is not needed as it caused linker errors as well.

llvm-svn: 228914