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

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


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19868 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b