Clean macros

[jlaumon]

The diff a is little bit messy because I moved a lot of stuff around, but here is the gist of it:

• all the macros are overridable (they're defined only if not already defined), and there is a system to include you own config header at the beginning of the file to override what you want
• all the platform specific bits are now wrapped inside a macro (all the #ifdef WIN32/GNUC in the main code are gone)
• all the macros are documented (at least a little bit)

This was a lot harder to do than I expected 😅, especially the parts with the virtual memory functions. But I'm pretty happy with the result, it should be very easy to port to new platforms now.

The main macros are: LTALLOC_VMALLOC, LTALLOC_VMFREE, and LTALLOC_VMALLOC_MIN_SIZE. If the plaform supports it, there are two more macros that you can define:

• LTALLOC_VMSIZE, which is meant to return size of a previous virtual mem allocation (if not available, eg. on Linux, ltalloc will store the sizes of the allocations internally in a ptrie).
  • LTALLOC_VMALLOC_ALIGNED, which allocates virtual memory with a specific alignment (Windows/Linux don't have a function for that, but some consoles do), and if not available, ltalloc has a slower method to make sure LTALLOC_VMALLOC end up returning a pointer with the right alignment.

[r-lyeh]

I am sure all the changes were done with good intentions but haven't checked all of them :) thanks!