m4/gst-plugin-docs.m4: add GST_PLUGIN_DOCS, which checks for everything needed to build the plugin docs (namely gtk-d...

Original commit message from CVS:
* m4/gst-plugin-docs.m4:
add GST_PLUGIN_DOCS, which checks for everything needed
to build the plugin docs (namely gtk-doc and pyxml); also
adds a new --enable-plugin-docs configure switch; will
set ENABLE_PLUGIN_DOCS conditional for use in Makefile.am
files (see #344039).