Don't muddle up cflags and ldflags

CFLAGS != LDFLAGS, using PLATFORM as global is bad.

Also for that bin file, well, /usr/bin is pretty common now, I don't know why some distros still prefer /usr/games.

DESTDIR != PREFIX, please do NOT put them together
Update Makefile

Add comments for extra flags specified by the user, keep the syntax ordered.
Merge pull request #12 from cicku/patch-2

* Separated PLATFORM into PLATFORM_CFLAGS and PLATFORM_LDFLAGS

* Moved binary from `/usr/games` to `/usr/bin`

* Made PREFIX not dependent on DESTDIR