[Bf-committers] with_system_glew / gles options on linux.

[Ray Molenkamp]

Not my regular platform, but I tend to help out the odd user with linux building errors, and this
one comes up rather frequently

In our cmakelist.txt we have an exception for linux that sets

option(WITH_SYSTEM_GLEW "Use GLEW OpenGL wrapper library provided by the operating system" ON)
option(WITH_SYSTEM_GLES "Use OpenGL ES library provided by the operating system"           ON)

compared to all other platforms where it is off.

I couldn't find a justification for it, but why are these options (unlike all other
WITH_SYSTEM_* options even on linux ) ON by default? Especially glew seems to be
problematic, even more so for blender 2.8 where we can't figure out if it's glew 1 or 2
at compile time so there's a runtime check in the final binary that'll prevent code
compiled against 1.x from running.

We ship with a known good copy in our source tree, why not use it by default?

--Ray