Use /usr/X11R6/include per default on OS X.

m4/opengl.m4

@@ -4,9 +4,10 @@ dnl
 AC_DEFUN([ALBERTA_OPENGL_CHECK],
 [# ALBERTA_OPENGL_CHECK start
 AC_REQUIRE([ALBERTA_X_WINDOW_SYSTEM])
+AS_IF([test -z "${x_includes}" -a "${OSX}" = true],[OGL_DFLTINCDIR=/usr/X11R6/include],[OGL_DFLTINCDIR=${x_includes}])
 ALBERTA_CHECK_PACKAGE([OpenGL],[GL],[glXMakeCurrent],
   [${x_libraries}],[${X_ALL_LIBS}],
-  [GL/gl.h],[${x_includes:+${x_includes}} ${includedir} ${oldincludedir}],[${X_CFLAGS}],
+  [GL/gl.h],[${x_includes:+${x_includes}} ${OGL_DFLTINCDIR} ${includedir} ${oldincludedir}],[${X_CFLAGS}],
   [optional])
 # ALBERTA_OPENGL_CHECK stop
 ])