Build Mmap.hh even if memory mapping is disabled

[Katharsas]

Basically ZenKit is kind of a nice file input library with its Read interface and Mmap class.

So i want to have the option to use Mmap for my own file loading independent of ZenKit usage. So even if ZenKit is configured to not use mmap, i can still use it directly in my own code.

Of course feel free to close if this does not fit very well, i can keep this change rebased on top of ZenKit branch myself if necessary.

[lmichaelis]

Heyo! I think it would be okay to expose the Mmap interface at all times, however this change is not enough to facilitate that.

We need to distinguish between "compiling with" and "using" memory mapping, so there need to be two different preprocessor defines. One which indicates whether memory-mapping support is available on the platform (which should be used instead of _ZK_WITH_MMAP in the Mmap.hh file) and one which indicates to the rest of ZenKit to actually use its memory-mapping capabilities (this needs to be a new preprocessor definition).

Because of this, it would probably be best to rebase this commit on the new mmap-free branch.

[Katharsas]

...

Yeah makes sense, i havent looked at the branch much yet so for now i just implemented it hopefully properly.

[Katharsas]

I could also change this back to being warning if you want