Compilation error on CentOS 6.5. Only using a subset of libs. Followed largely https://trac.ffmpeg.org/wiki/CentosCompilationGuide. I use a slightly  different prefix. Had to modify in Section FFmpeg
> PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig"
> export PKG_CONFIG_PATH
to
> export PKG_CONFIG_PATH="$HORNET_PREFIX/lib/pkgconfig".

This gives me:
echo $PKG_CONFIG_PATH
/home/bgeese2s-local/hornet_dist/lib/pkgconfig

And furthermore:
pkg-config --cflags opus x264
-I/home/bgeese2s-local/hornet_dist/include/opus -I/home/bgeese2s-local/hornet_dist/include

However i get libx264 not found:
./configure --prefix="$HORNET_PREFIX" --extra-cflags="-I$HORNET_PREFIX/include" --extra-ldflags="-L$HORNET_PREFIX/lib" --enable-gpl --enable-libopus --enable-libx264
ERROR: libx264 not found

Here is the bottom of config.log:
BEGIN /tmp/ffconf.zcAFkqRZ.c
    1   #include <opus_multistream.h>
    2   long check_opus_multistream_decoder_create(void) { return (long) opus_multistream_decoder_create; }
    3   int main(void) { return 0; }
END /tmp/ffconf.zcAFkqRZ.c
gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -I/home/bgeese2s-local/hornet_dist/include -std=c99 -fomit-frame-pointer -pthread -I/home/bgeese2s-local/hornet_dist/include/opus -L/home/bgeese2s-local/hornet_dist/lib -c -o /tmp/ffconf.SLJs6mAV.o /tmp/ffconf.zcAFkqRZ.c
gcc -L/home/bgeese2s-local/hornet_dist/lib -Wl,--as-needed -I/home/bgeese2s-local/hornet_dist/include/opus -L/home/bgeese2s-local/hornet_dist/lib -o /tmp/ffconf.bfsabQjg /tmp/ffconf.SLJs6mAV.o -lopus -lm -pthread -lz -lrt
check_lib x264.h x264_encoder_encode -lx264
check_header x264.h
check_cpp
BEGIN /tmp/ffconf.zcAFkqRZ.c
    1   #include <x264.h>
    2   int x;
END /tmp/ffconf.zcAFkqRZ.c
gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -I/home/bgeese2s-local/hornet_dist/include -std=c99 -fomit-frame-pointer -pthread -I/home/bgeese2s-local/hornet_dist/include/opus -E -o /tmp/ffconf.SLJs6mAV.o /tmp/ffconf.zcAFkqRZ.c
In file included from /tmp/ffconf.zcAFkqRZ.c:1:
/home/bgeese2s-local/hornet_dist/include/x264.h:36:4: warning: #warning You must include stdint.h or inttypes.h before x264.h
check_func x264_encoder_encode -lx264
check_ld cc -lx264
check_cc
BEGIN /tmp/ffconf.zcAFkqRZ.c
    1   extern int x264_encoder_encode();
    2   int main(void){ x264_encoder_encode(); }
END /tmp/ffconf.zcAFkqRZ.c
gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -I/home/bgeese2s-local/hornet_dist/include -std=c99 -fomit-frame-pointer -pthread -I/home/bgeese2s-local/hornet_dist/include/opus -c -o /tmp/ffconf.SLJs6mAV.o /tmp/ffconf.zcAFkqRZ.c
gcc -L/home/bgeese2s-local/hornet_dist/lib -Wl,--as-needed -o /tmp/ffconf.bfsabQjg /tmp/ffconf.SLJs6mAV.o -lx264 -L/home/bgeese2s-local/hornet_dist/lib -lopus -lm -pthread -lz -lrt
/home/bgeese2s-local/hornet_dist/lib/libx264.a(opencl.o): In function `x264_opencl_close_library':
opencl.c:(.text+0x5f1): undefined reference to `dlclose'
/home/bgeese2s-local/hornet_dist/lib/libx264.a(opencl.o): In function `x264_opencl_load_library':
opencl.c:(.text+0x643): undefined reference to `dlopen'
opencl.c:(.text+0x65c): undefined reference to `dlsym'
opencl.c:(.text+0x676): undefined reference to `dlsym'
opencl.c:(.text+0x690): undefined reference to `dlsym'
opencl.c:(.text+0x6aa): undefined reference to `dlsym'
opencl.c:(.text+0x6c4): undefined reference to `dlsym'
/home/bgeese2s-local/hornet_dist/lib/libx264.a(opencl.o):opencl.c:(.text+0x6de): more undefined references to `dlsym' follow
/home/bgeese2s-local/hornet_dist/lib/libx264.a(opencl.o): In function `x264_opencl_load_library':
opencl.c:(.text+0x944): undefined reference to `dlclose'
/home/bgeese2s-local/hornet_dist/lib/libx264.a(opencl.o): In function `x264_opencl_lookahead_init':
opencl.c:(.text+0x1916): undefined reference to `dlopen'
opencl.c:(.text+0x1931): undefined reference to `dlsym'
opencl.c:(.text+0x1945): undefined reference to `dlsym'
opencl.c:(.text+0x195c): undefined reference to `dlsym'
opencl.c:(.text+0x1970): undefined reference to `dlsym'
opencl.c:(.text+0x19a6): undefined reference to `dlclose'
collect2: ld returned 1 exit status
ERROR: libx264 not found