Port AC_FUNC_MMAP to more-modern systems

Port AC_FUNC_MMAP to more-modern systems

* lib/autoconf/functions.m4 (AC_FUNC_MMAP): Don’t call
getpagesize, as it’s tricky to configure, modern POSIX doesn’t
require it, and calling it without including <unistd.h> provokes a
compile-time error on modern systems.  Instead, rework the test
to not need getpagesize.  Add a FIXME comment for unnecessary
tests; I don't want to remove them now as we're too close to
a release.  Remove long-obsolete comment about GNU grep.