Using read/write/exec causes an error (or possibly abort) on systems
with W^X policies (like OpenBSD and NetBSD have by default).
Since the heap does not need to be executable, just use read/write.
When mmap fails it returns MAP_FAILED, not NULL. POSIX does not
define the value for MAP_FAILED, but on at least the BSDs and Linux
its value is ((void *) -1).
Try to fix win32 port. Now it runs both on Win32/Win64.
Win64 port currently depends on 128bits arithmetic thus it does not run on
MSVC.
Makefile now have EXCLUDE_POSIX_LIBS knob to exclude posix related library
from build.
Introduce msys PLATFORM for Makefile.detect to use MSYS's POSIX
emulation layer. It is intended for linking against MSYS tools; it is
not for embedding to Win32 applications.
