Changes made for Mac OS compatibility

[aferenc]

Some relatively minor tweaks made to make the source code compile on MacOS, while trying to remain compatible with both Windows and Linux.

[Espyo]

Adding a bit of context before approving:

• allegro_main-5 in the makefile is needed for Mac OS compilation, and doesn't break anything when compiling for Linux; I've tested.
• The change of the font name from "main" to "standard" is because Allegro's got a macro when compiling for Mac OS named "main".
• __MACH__ and DARWIN don't seem to be standard. Not quite sure where I got them from.
• Under Mac OS, it seems fullscreen causes it to freeze and lock up the system. For the time being, fullscreen functionalities there are to be locked to "windowed fullscreen".