Initial patches to compile under Linux

These patches enable the compilation of the current version under Linux (or other POSIX-compatible OS) without errors and warnings.

The first commit mostly replaces the Windows File IO with POSIX equivalents and masks other windows-specific parts (e.g., window management and dialogs) on Linux. In two cases, I kept the Windows version as the file handle is still used in the original code.
The second commit mostly removes "static" from "enum" declarations and includes additional headers - mostly "string.h" in order to fix remaining warnings.
The third commit adds an initial CMakeLists.txt file that enables building the project on Linux. One should also be able to generate the necessary build/projects for Windows and Mac with this file and cmake.

I only tested this under Linux but every POSIX function should be available under Windows too and should not create problems for Windows developers. Unfortunately, I don't have access to a Windows PC with build environment the really proof this claim. I'd appreciate feedback if anyone has some spare time to test it.

I know that these patches don't help much at the moment but I'd propose to stick to the portable code in order to minimize the work later.