Fix Linux X11 conflits with Qt 5.9

[edubart]

This solves https://github.com/ValveSoftware/vogl/issues/222

X11 header definitions was conflicting with Qt headers, a solution was to undef the conflicting symbols just after including vogl_common.h and avoid to use them

Now I am able to built with Qt 5.9 and GCC 7.2 on ArchLinux

[underdoeg]

thanks, this works for me