get rid of compiler warnings

I get the following compilation warnings.

```
../deps/libmagic/src/cdf.c: In function ‘cdf_read’:
../deps/libmagic/src/cdf.c:299:2: warning: implicit declaration of function ‘pread’ [-Wimplicit-function-declaration]
```

I'm assuming the first one is related to adding support for windows. All *nix architectures that define `HAVE_PREAD` should get this warning because `<unistd.h>` no longer is included. And windows doesn't get this warning because `HAVE_PREAD` is undefined. The solution is to remove the `HAVE_PREAD` constant altogether and simply use the same function signature for all architectures.

Please correct me if I'm wrong and have missed something here.

```
../deps/libmagic/pcre/pcre_compile.c: In function ‘add_to_class’:
../deps/libmagic/pcre/pcre_compile.c:3487:30: warning: ‘od’ may be used uninitialized in this function [-Wmaybe-uninitialized]
```

This warning is related to `od` not being initialized _if_ `get_othercase_range()` returns from: https://github.com/mscdex/mmmagic/blob/master/deps/libmagic/pcre/pcre_compile.c#L2935-L2937, but since `get_othercase_range()` returns a non zero value the value of `od` is irrelevant, hence we can just set the initial value of `od` to anything, e.g. 0.

Unfortunately there aren't any tests I can run to make sure this is completely ok, so need your feedback.