Bug 710376 - Make dist problems with pdf-import

 - the directoy must be listed in plug-ins/Makefile.am
 - usage of POPPLER_CFLAGS
make check was choking on fill-rule in shape files
Bug 710379 - [warningectomy] no AC_LANG_SOURCE call detected in body

Probably we can get rid of the detailed PNG source check nowadays,
but just adding AC_LANG_SOURCE is easier.