flags that autoconf hasn’t checked first.

disabled until the engine code itself is ported to SDL2.

to external so as to avoid some autocompletion collisions with "configure".

that the OpenGL framework on OS X provides GL version 2 at least.

when building for the PS3 on a Unix system.

it understands V=0 and V=1.

accept them as variable names.

nothing for now except ensure that every makefile includes common.am.