Add -lm to x264 check, it is needed on most systems.

taken from the DOS/DJGPP port by Michael Kostylev, mik niipt ru

Originally committed as revision 12178 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1646,7 +1646,7 @@ enabled libmp3lame && require  LAME lame/lame.h lame_init -lmp3lame -lm
 enabled libnut     && require  libnut libnut.h nut_demuxer_init -lnut
 enabled libtheora  && require  libtheora theora/theora.h theora_info_init -ltheora -logg
 enabled libvorbis  && require  libvorbis vorbis/vorbisenc.h vorbis_info_init -lvorbisenc -lvorbis -logg
-enabled libx264    && require  x264 x264.h x264_encoder_open -lx264
+enabled libx264    && require  x264 x264.h x264_encoder_open -lx264 -lm
 enabled libxvid    && require  Xvid xvid.h xvid_global -lxvidcore
 enabled mlib       && require  mediaLib mlib_types.h mlib_VectorSub_S16_U8_Mod -lmlib