The point of the matter is that posix_memalign() is not allowed in this
context according to the relevant standards, so we use mmap() instad.

`mprotect` can only be used for `mmap`'ed memory

Makes testdebugallocator pass on OS X, too.

See merge request !43
(cherry picked from commit 6467775c)

Signed-off-by: Steffen Müthing <muething@dune-project.org>