Corrected ordering of characters in INT_ID macro.

Sorry, this will break save games again. In short, a multi character
constant 'ABCD' should have 'A' as the highest bits, and 'D' as the lowest
bits. Before it was the other way around.
Added _PORTABLE_VERSION to SP code.

This allows portable versions of the SP code to be built (portable as in,
the home/user directory is not used for reading/writing files as in the
new system). I've also added a CMake switch to allow this to be toggled
(BuildPortableVersion).