gpac-libgpac compile failure header file missing?

[David Southwell]

make stops with:

compositor/events.c: In function 'load_text_node':
compositor/events.c:239: warning: passing argument 3 of 'gf_utf8_wcstombs' 
from incompatible pointer type
compositor/events.c:247: warning: passing argument 3 of 'gf_utf8_wcstombs' 
from incompatible pointer type
cc -O3  -O2 -fno-strict-aliasing -pipe  -fPIC -pthread -fno-strict-aliasing -Wno-pointer-sign -I/usr/ports/multimedia/gpac-libgpac/work/gpac/include  -I../ -DGPAC_HAVE_CONFIG_H  -fPIC -DPIC -c 
compositor/font_engine.c -o compositor/font_engine.opic
cc -O3  -O2 -fno-strict-aliasing -pipe  -fPIC -pthread -fno-strict-aliasing -Wno-pointer-sign -I/usr/ports/multimedia/gpac-libgpac/work/gpac/include  -I../ -DGPAC_HAVE_CONFIG_H  -fPIC -DPIC -c 
compositor/hardcoded_protos.c -o compositor/hardcoded_protos.opic
cc -O3  -O2 -fno-strict-aliasing -pipe  -fPIC -pthread -fno-strict-aliasing -Wno-pointer-sign -I/usr/ports/multimedia/gpac-libgpac/work/gpac/include  -I../ -DGPAC_HAVE_CONFIG_H  -fPIC -DPIC -c 
compositor/mesh.c -o compositor/mesh.opic
In file included from compositor/mesh.c:33:
compositor/gl_inc.h:45:19: error: GL/gl.h: No such file or directory
gmake: *** [compositor/mesh.opic] Error 1
*** Error code 2

Stop in /usr/ports/multimedia/gpac-libgpac.

seems to be looking for a missing header file