Make pkgconfig_generate() fill Libs with the required linking flags

if the "shared" var has not been explicitely enabled, rather than if
it has been explicitly disabled.

This way is not necessary to explicitly disable shared libraries in
order to have a pc file usable with only static libraries.

Originally committed as revision 19868 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2796,10 +2796,10 @@ includedir=$incdir
 Name: $name
 Description: $comment
 Version: $version
-Requires: $(disabled shared && echo $requires)
+Requires: $(enabled shared || echo $requires)
 Requires.private: $(enabled shared && echo $requires)
 Conflicts:
-Libs: -L\${libdir} -l${shortname} $(disabled shared && echo $libs)
+Libs: -L\${libdir} -l${shortname} $(enabled shared || echo $libs)
 Libs.private: $(enabled shared && echo $libs)
 Cflags: -I\${includedir}
 EOF