compile error with --enable-opengl on osx

[ponpon]

compile error arise with --enable-opengl on osx, where there is glx.h file of macports.
the error doesnt appear, removing glx.h file.
glx.h is included in ffmpeg/libavdevice/opengl_enc.c

glx.h and opengl is installed when installing mesa with macports. mesa is used by xz, pkgconfig, flex, bison, indent, autoconf, automake, libtool, py27-libxml2, xorg-glproto, xorg-dri2proto, xorg-libxcb, xorg-libX11, xorg-libXext, xorg-libXdamage, xorg-libXfixes, xorg-libXi, xorg-libXmu, xorg-libXxf86vm.
probably, many macports users abandon to use --enable-opengl.

./configure --cc=clang-mp-3.7 --cxx=clang++-mp-3.7  --extra-cflags="  -I/opt/local/include "  --extra-ldflags=" -L/opt/local/lib "  --enable-opengl && make -j 8 
.
.
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CC	libavfilter/af_amerge.o
libavdevice/opengl_enc.c:494:30: error: use of undeclared identifier
      'glCompileShader'
    procs->glCompileShader = glCompileShader;
                             ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
4 warnings and 20 errors generated.
make: *** [libavdevice/opengl_enc.o] Error 1
make: *** Waiting for unfinished jobs....

./configure --cc=clang-mp-3.7 --cxx=clang++-mp-3.7  --extra-cflags=" -ferror-limit=0  -I/opt/local/include "  --extra-ldflags=" -L/opt/local/lib "  --enable-opengl  && make -j 8 
.
.
.
libavdevice/opengl_enc.c:499:70: error: use of undeclared identifier 'glVertexAttribPointer'
    procs->glVertexAttribPointer = (FF_PFNGLVERTEXATTRIBPOINTERPROC) glVertexAttribPointer;
                                                                     ^
CC	libavfilter/af_afade.o
CC	libavfilter/af_afftfilt.o
4 warnings and 24 errors generated.
make: *** [libavdevice/opengl_enc.o] Error 1
make: *** Waiting for unfinished jobs....