This check in updates SDL_atomic.h to reflect the new set of atomic operations in 32 and 64 bit form.
It also update configure.in to compile the linux version of the library. The three versions are all dummies
implementations that do nothing. They are being checked in as place holders. Mostly, I just wanted to get
place holders and the configure.in checked in.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403667