install libav(codec|format).pc/libpostproc.pc for pkg-config (original patch from <j at v2v.cc>)

Originally committed as revision 4276 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -478,6 +478,8 @@ install-headers:
 	               $(SRC_PATH)/libavcodec/common.h \
 	               $(SRC_PATH)/libavcodec/rational.h \
                 "$(prefix)/include/ffmpeg"
+	install -d $(libdir)/pkgconfig
+	install -m 644 ../libavcodec.pc $(libdir)/pkgconfig
 
 #
 # include dependency files if they exist