Glew.obj is one of the more expensive targets we have to build clocking in
at 34.907 seconds.

The root cause [1] is the msvc inliner is not super thrilled with the
glewIsSupported supported function, this patch changes the buildflags
of extern_glew to optimize for size rather than speed.

Given glew is only in play at initalization time there will be no
performance impact.

Time to build glew.obj

Before: 34.907s
After :  1.961s

[1] https://developercommunity.visualstudio.com/content/problem/732941/slow-compilation-of-glewc-for-visual-studio-2019-x.html

Differential Revision: https://developer.blender.org/D6656

Reviewers: brecht, sergey, fclem