Add platform and compiler defines

[billyquith]

Current Gwork uses the default OS and compiler defines, e.g. _WIN32 etc. This can be problematic where you get setups that are a mix of these, e.g. MinG Win.

Add, in a platform header, OS, and compiler detection and defines and replace the current macros used.

[billyquith]

Work:

• Create a header file (or put in Config.h) in the platform layer and then replace the various current defines.
• Prefix all the macros with GWK_, maybe, GWK_OS_*, GWK_PLATFORM_*, GWK_COMPILER_*. Platforms can be cross-platform, so Allegro might have different OS but be same platform.