Initial attempt at --steam config flag

The flag right now should link against steam_api, include the necessary
header files, and define STEAM for the preprocessor.
Add steam-sdk makefile

The steam sdk itself isn't included; you'll have to download that and
place it appropriately.
Update INSTALL documentaion; make it markdown too
Init / shutdown steam api appropriately
Add steam achievement command
Error messages in achievement command
Add steamsetachieve to prototypes
Remove Steam Makefile, letting it build properly. Add some better OSX build instructions
Merge branch 'SteamApi' into SteamAchieves
Remove useless debug line
Fix steam makefile and configure

Makefile was looking for header in the wrong directory which caused a
fresh compile every time.

Configure script didn't include the steamlib directory which failed if
no other libs were bundled.
Remove unneeded fix-timestamps line
Merge branch 'SteamApi' into SteamAchieves
Merge branch 'master' into SteamApi
Merge branch 'SteamApi' into SteamAchieves
Fix linking against steam_api.dll on windows
Merge pull request #76 from sekaiproject/SteamAchieves

Steam Achievements
Backport SDL2 fix from upstream

Commit: http://hg.libsdl.org/SDL/rev/e217ed463f25

Description:
Fixed a build error in SDL_platform.h when using Code Blocks and MinGW
Merge better_messages and master manually
Import PonscripterMessage header file