Pkgconfig metafile for plugins

Thanks to braden@endoframe.com for the file.
Bug#174915 r=cls a=roz+moc

build/unix/mozilla-plugin.pc.in

@@ -0,0 +1,10 @@
+prefix=%prefix%
+exec_prefix=%exec_prefix%
+libdir=%libdir%
+includedir=%includedir%
+
+Name: Mozilla Plug-In API
+Description: Mozilla Plug-In API
+Version: %MOZILLA_VERSION%
+Requires: mozilla-xpcom = %MOZILLA_VERSION%
+Cflags: -I${includedir}/java -I${includedir}/plugin