unix: Simply check for the presence of mmap

There is little point to check if mmap is working on the system
that is building the software. Simply check if mmap is present
or not. This avoids running into issues around mmap_min_addr.

This is a backport of pharo-project/pharo-vm@43461db
It is required on Ubuntu 14.04 (see step 2 in previous commit)
Other reference by the original author:

"I have not fully read the macro but I think the AC_FUNC_MMAP runs into the mmap_min_addr restriction. But as most people use a binary version of the VM there is little point to check that the system that built the software had a working version (and since the late 90s mmap generally works).

https://wiki.debian.org/mmap_min_addr"

platforms/unix/config/configure.ac

@@ -298,7 +298,7 @@ AC_CHECK_FUNC(_dyld_present, [
 
 AC_CHECK_FUNCS(snprintf __snprintf,[break])
 
-AC_FUNC_MMAP
+AC_CHECK_FUNCS([mmap])
 AC_FUNC_ALLOCA
 AX_HAVE_EPOLL
 AX_HAVE_EPOLL_PWAIT