Fix 'make check' when mono is not installed

When the xbuild_12 profile is verified, the corlib from the net_4_5
profile should be in the MONO_PATH, otherwise the verification will
fail with:

The assembly mscorlib.dll was not found or could not be loaded.
It should have been installed in the `.../mono/4.5/mscorlib.dll' directory.
Merge pull request #1002 from ranma42/make-check-no-install

Fix 'make check' when mono is not installed