configure: fix check for x264

libx264 recently started mangling the name of x264_encoder_open() to
prevent version mismatches, breaking our test.  Checking for another
function instead makes it work again.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19754 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -2283,7 +2283,7 @@ enabled libschroedinger && add_cflags $(pkg-config --cflags schroedinger-1.0) &&
 enabled libspeex   && require  libspeex speex/speex.h speex_decoder_init -lspeex
 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  libx264 x264.h x264_encoder_open -lx264 -lm &&
+enabled libx264    && require  libx264 x264.h x264_encoder_encode -lx264 -lm &&
                       { check_cpp_condition x264.h "X264_BUILD >= 65" ||
                         die "ERROR: libx264 version must be >= 0.65."; }
 enabled libxvid    && require  libxvid xvid.h xvid_global -lxvidcore