malloc/realloc/free allocator, I moved it into physfs.c as a default, which
is used if the platform layer doesn't offer a platform-specific default
allocator, which none do at this point, but will soon.
#ifdefs...this will make it easier to "embed" this library in other projects
or use a different build system: just push everything through the compiler
with preprocessor defines for the parts you want/need...platform modules are
determined automatically without the build system needing to intervene, so
you just have to #define the archivers, etc that you want.
Ryan C. Gordon