[CMake] Adjust the variables set by LLVMConfig.cmake

When using LLVMConfig.cmake from an installed toolchain in order to build a
loadable pass using add_llvm_loadable_module LLVM_ENABLE_PLUGINS and
LLVM_PLUGIN_EXT must be set. Also make LLVM_DEFINITIONS be set to what it
actually is.

Differential Revision: http://reviews.llvm.org/D13214

llvm-svn: 248884