Make pkgconfig_generate() explicitly return 0 in instead of returning

Make pkgconfig_generate() explicitly return 0 in instead of returning
without value when the target library is disabled.

If it does not explicitly return 0, when the last library is disabled
(swscale), the final "configure" exit value is 1, even if the
configure script is successfully executed. So it breaks scripts that
invoke configure and rely on 0 for success and 1 for failure.

Patch by Jean-Daniel Dupas reverse(<org.shadowlab@devlists>).

Originally committed as revision 24585 to svn://svn.ffmpeg.org/ffmpeg/trunk