Build fix for moglcore on NeuroDebian et al.

Use a #ifdef PTB_USE_WAFFLE instead of #ifdef LINUX to only enable the
special GLEW init code path for Linux if libwaffle is in use and we
therefore also employ our own patched GLEW library. This is important
because only our own GLEW version exports the function
glewContextInit(), but not a standard libGLEW v1.9, as used by
NeuroDebian/Debian/Ubuntu builds.

Default to classic "textbook" path which calls glewInit().
-> Should fix NeuroDebian build failure, does not require a new
standard PTB release.