We require pkg-config to check for, for example, GLib.

If the check for pkg-config fails, quit immediately; don't leave the
failure message for later, as the error message in question might not be
as blunt.

Change-Id: I530e8b62f5adff228ae8ff6f9798ac538c2ad684
Reviewed-on: https://code.wireshark.org/review/6084
Reviewed-by: Guy Harris <guy@alum.mit.edu>