AC_FUNC_MMAP lacks <unistd.h> include?

[Sam James]

Matt noticed in https://bugs.gentoo.org/898816 that
it appears AC_FUNC_MMAP is missing a <unistd.h> include:
```
> configure:16692: checking for working mmap
> configure:16844: x86_64-pc-linux-gnu-clang -o conftest -O2
> -march=native -pipe -Werror=implicit-function-declaration
> -Werror=implicit-int -Werror=incompatible-function-pointer-types
> -std=c99  -Wl,-O1 -Wl,--as-needed conftest.c  >&5
> conftest.c:138:14: error: call to undeclared function 'getpagesize';
> ISO C99 and later do not support implicit function declarations
> [-Werror,-Wimplicit-function-declaration]
>   pagesize = getpagesize ();
>              ^
> 1 error generated.
```

It looks like this is still the case in master as well.

best,
sam

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