GL/gl.h, Qt5 and arm: FTBFS

[Rebecca N. Palmer]

This problem has been seen before in Qt4, where Debian briefly tried 
using GLES2 on arm* with 4.7.3 then changed back, but Ubuntu kept using 
it, failed build example: 
https://launchpad.net/ubuntu/+source/openscenegraph/3.2.0~rc1-4/+build/5615979

The changelogs note that GLES was used, but not why.

In Qt4, changing between GL and GLES breaks ABI (#632602); I don't know 
if this is also the case in Qt5 (which would require a libqt5opengl5 -> 
libqt5opengl5a transition).