common.h has some #define when doing win32 build to workaround a few
missing functions on these systems. However, since mingw32 has some
of these, this causes either warnings about redefining preprocessor
symbols or wreak havoc in mingw headers trying to use these symbols.
This commit wraps these symbols in an #ifndef __MINGW32__ to avoid
using them on this platform.
The OpenGL renderer isn't really useful right now, its not quite up
to date, its not really faster than software and it only supports a limited
subset of drivers. So, lets disable it for now.
Long term opengl rendering of the 2d part of spice is important if we want
to combine 2d and 3d rendering (say if spice adds opengl support in the
protocol). But until then this is isn't useful for normal use.
