glsl: use -O1 optimization for builtin_functions.cpp with MinGW

glsl: use -O1 optimization for builtin_functions.cpp with MinGW

Some versions of MinGW-w64 such as 5.3.1 and 6.2.0 produce bad code
with -O2 or -O3 causing a random driver crash when running programs
that use GLSL.  Most Mesa demos in the glsl/ directory trigger the
bug, but not the fragcoord.c test.

Use a #pragma to force -O1 for this file for later MinGW versions.
Luckily, this is basically one-time setup code.  I suspect the bug
is related to the sheer size of this file.

This should let us move to newer versions of MinGW-w64 for Mesa.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>