Don't fail with an error if files not found.
Fix numerous warnings
Merge branch 'master' of https://github.com/bradharding/doomretro

Conflicts:
	src/am_map.c
	src/g_game.c
Rename expansion enum values.

m_menu.c uses 'exp1', 'exp2' as enum values, but 'exp2' is a library
function on OS X.