avoid duplicate -lm in .pc files

configure

@@ -3428,7 +3428,7 @@ EOF
 }
 
 pkgconfig_generate libavutil "Libav utility library" "$LIBAVUTIL_VERSION" "$LIBM"
-pkgconfig_generate libavcodec "Libav codec library" "$LIBAVCODEC_VERSION" "$extralibs $LIBM" "libavutil = $LIBAVUTIL_VERSION"
+pkgconfig_generate libavcodec "Libav codec library" "$LIBAVCODEC_VERSION" "$extralibs" "libavutil = $LIBAVUTIL_VERSION"
 pkgconfig_generate libavformat "Libav container format library" "$LIBAVFORMAT_VERSION" "$extralibs" "libavcodec = $LIBAVCODEC_VERSION"
 pkgconfig_generate libavdevice "Libav device handling library" "$LIBAVDEVICE_VERSION" "$extralibs" "libavformat = $LIBAVFORMAT_VERSION"
 pkgconfig_generate libavfilter "Libav video filtering library" "$LIBAVFILTER_VERSION" "$extralibs"