Blender Git Loki

Docs: Add preprocessor define for doxygen

Doxygen by default leaves out any functions inside
#ifdef blocks that it thinks are disabled.

This change adds a DOXYGEN symbol, so you can
still get the functions included in the
documentation even if the #ifdef would
have normally excluded them.

before

#if defined(_WIN32)

after

#if defined(_WIN32) || defined(DOXYGEN)

Patch provided by Campbell Barton on chat.