configure: Add missing libm library dependencies to .pc files.

This unbreaks static compilation using pkg-config on systems in need of -lm.
Based on an mplayer2 patch by Uoti Urpala <uau@mplayer2.org>

Signed-off-by: Diego Biurrun <diego@biurrun.de>

configure

@@ -2840,7 +2840,7 @@ for thread in $THREADS_LIST; do
     fi
 done
 
-check_lib math.h sin -lm
+check_lib math.h sin -lm && LIBM="-lm"
 enabled vaapi && require vaapi va/va.h vaInitialize -lva
 
 check_mathfunc exp2
@@ -3427,10 +3427,10 @@ Cflags: -I\${includedir}
 EOF
 }
 
-pkgconfig_generate libavutil "Libav utility library" "$LIBAVUTIL_VERSION"
-pkgconfig_generate libavcodec "Libav codec library" "$LIBAVCODEC_VERSION" "$extralibs" "libavutil = $LIBAVUTIL_VERSION"
+pkgconfig_generate libavutil "Libav utility library" "$LIBAVUTIL_VERSION" "$LIBM"
+pkgconfig_generate libavcodec "Libav codec library" "$LIBAVCODEC_VERSION" "$extralibs $LIBM" "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" "$extralibs"
 pkgconfig_generate libpostproc "Libav post processing library" "$LIBPOSTPROC_VERSION" "" "libavutil = $LIBAVUTIL_VERSION"
-pkgconfig_generate libswscale "Libav image rescaling library" "$LIBSWSCALE_VERSION" "" "libavutil = $LIBAVUTIL_VERSION"
+pkgconfig_generate libswscale "Libav image rescaling library" "$LIBSWSCALE_VERSION" "$LIBM" "libavutil = $LIBAVUTIL_VERSION"