Handle changing of LLVM_ENABLE_FFI.

If someone first configure build with LLVM_ENABLE_FFI=1 and then turn it
off, the build will fail in lib/ExecutionEngine/Interpreter because
Interpreter will try still to #include <ffi/ffi.h>, but there are no
include_directories(${FFI_INCLUDE_DIR}) now.

This patch unset()'s HAVE_FFI_H and HAVE_FFI_FFI_H from cache file if
LLVM_ENABLE_FFI=0. This forces CMake to update config.h.

Patch by arrowdodger!

llvm-svn: 128769