defining __GLIBC__, as done already for Linux and Hurd (cf. bug #5613).
Also includes some more __GNU__ checks on top of those made in
commit ade4bf0907.
Thanks Petr Salinger (Debian bug #400869).
Implement glGetProgramStringARB and glGetProgramStringNV. With these
functions implemented, GL_ARB_{vertex,fragment}_program,
GL_NV_{vertex,fragment}_program, and related extensions can be enabled.