MakeMaker.pm: optionally use ExtUtils::PkgConfig to detect system libffi

Installing ExtUtils::PkgConfig requires that pkg-config be in the PATH, which
breaks down on some systems that might otherwise work. This allows one to use
the system libffi on cygwin, where the libffi headers are not in /usr/include.

It may help on some other platforms as well.
Changes: update