m4/gst-check.m4: add GST_PKG_CHECK_MODULES, which in the normal case of checking for a dependency lib for a plug-in o...

Original commit message from CVS:

* m4/gst-check.m4:
add GST_PKG_CHECK_MODULES, which in the normal case of checking
for a dependency lib for a plug-in only needs two arguments
to do the right thing.
* m4/gst-feature.m4:
clean up output a little of feature checking; also deal with
non-plug-in feature checks
* m4/Makefile.am:
* m4/gst-gstreamer.m4:
remove this file; it's a useless check