Portability

[zenorogue]

Various commits intended to improve portability, in various ways.

Note: I am cross-compiling the Windows version on Linux using x86_64-w64-mingw32-g++. So this is 64-bit. I am not sure how to make cmake work (I am compiling using my own scripts instead which I have not included in the PR).

• The saves in the Linux and Windows versions I compiled were not compatible, because on Linux "long" is 64-bit, while in Windows, at least according to the x86_64-w64-mingw32-g++ compiler, it is 32-bit, so Windows would read 32 bit of a Linux long and the next 32 bits would be assumed to refer to something else. I have fixed this by making save.h use 64 bits anyway when recording "long" or "ulong". An alternative solution would be redefine "ulong" to be 64-bit and replace all occurrences of "long" with a new 64-bit type.
• The Linux version was configured to install itself in the system -- added a cmake option PORTABLE_BUILD to create a "portable build" working from a single directory. Also this option uniformizes the highscore filename between Linux and Windows. It was mostly for Steam but I guess it could also be used if you wanted to launch IVAN from a USB stick that you use on both Linux and Windows, and keep user data between them.
• There was a problem with festring and fearray allocating an int* before the actual content and then treating it as ulong, which did not work when int* and ulong have different sizes. (Weird that this worked on Wine for me. It crashed on actual Windows though.)
• Also it appears that _findfist crashes in w64-mingw32 (even in my minimal program) -- I have added an option USE_OPENDIR to use the Unix standard functions opendir/readdir which work there.