Fails to build without CPLUS_INCLUDE_PATH

When trying to build against mesa installed in $installdir, building glmark2 fails if CPLUS_INCLUDE_PATH is not set to $installdir/include , with:

[117/124] cxx: src/libmatrix/program.cc -> build/src/libmatrix/program.cc.3.o
In file included from ../src/libmatrix/gl-if.h:17:0,
                 from ../src/libmatrix/program.cc:17:
/home/darxus/source/glmark2/src/gl-headers.h:28:19: fatal error: GL/gl.h: No such file or directory
compilation terminated.
[118/124] cxx: src/libmatrix/shader-source.cc -> build/src/libmatrix/shader-source.cc.3.o
Waf: Leaving directory `/home/darxus/source/glmark2/build'
Build failed

I believe this is a bug.

I'm building with: ./waf configure --with-flavors=wayland-glesv2,wayland-gl --prefix=$installdir

I have a number of other variables set which are adequate for most things to find what they need in $installdir, as documented in the "Setting up the environment" section of http://wayland.freedesktop.org/building.html

(Other than that, I verified this works on wayland now, thanks.)