include: posix.mman += MAP_FAILED
include: posix.mman += MAP_FAILED

mmap returns void* and indicates failure via MAP_FAILED that is defined
to be ((void *)-1). Every mmap call has to be checked for failure and
thus posix.mman with mmap, but without MAP_FAILED is not very useful.

We cannot declare MAP_FAILED as enum, because enum assumes int. However
Cython documentation says

	4. If the header file uses macros to define constants, translate
	   them into a normal external variable declaration. ...

	(https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#referencing-c-header-files)

So add proper declaration for MAP_FAILED to make posix.mman.mmap usable.

(cherry picked from commit 6725304967f4c4235cb0113c6f0e8b56bc683478)

cython/cython,scoder/cython,cython/cython,da-woods/cython,scoder/cython,scoder/cython,scoder/cython,da-woods/cython,da-woods/cython,cython/cython,cython/cython,da-woods/cython