configure WARNING

[William Bendick]

I get the following from a configure script i am working on:

checking GL/gl.h usability... yes
checking GL/gl.h presence... no
configure: WARNING: GL/gl.h: accepted by the compiler, rejected by the 
preprocessor!
configure: WARNING: GL/gl.h: proceeding with the preprocessor's result
configure: WARNING:     ## ------------------------------------ ##
configure: WARNING:     ## Report this to address@hidden ##
configure: WARNING:     ## ------------------------------------ ##
checking for GL/gl.h... no

I do have the header file:

<address@hidden:3> ls /usr/pkg/include/GL/
gl.h          glu.h         glutf90.h     glxext.h      xmesa_x.h
gl_mangle.h   glu_mangle.h  glx.h         osmesa.h      xmesa_xf86.h
glext.h       glut.h        glx_mangle.h  xmesa.h
<address@hidden:4> 

Can you explain this? 

Thank you,
William Bendick