Review possible dead code around EXV_HAVE_MMAP

[piponazo]

In #2219 (comment) we discussed about the possibility of having some dead code in our codebase around the EXV_HAVE_MMAP definition:

As far as I can see we have the following preprocessor conditions:
```cpp
#if defined EXV_HAVE_MMAP && defined EXV_HAVE_MUNMAP
...
#elif defined WIN32 && !defined __CYGWIN__
...
#else
...  <-- This is the area where we could have dead code
#endif

We need to find out if there is any UNIX platform without the system header sys/mman.h. If it does not exist, we could consider certain areas of the codebase as dead-code and remove them.

[kmilos]

See also #2387

[kevinbackhouse]

My proposal in #2387 is to put a #error in the "dead code" area. Super-easy to revert if it turns out that somebody needs that code.