include/prereq-build.mk: macOS cross build improvements

include/prereq-build.mk: macOS cross build improvements

This updates prereq-build.mk to find a suitable realpath utility, and
adds another place to look for a suitable getopt utility.

realpath has been used most notably by scripts/ipkg-build since
commit bb95be9265a9 ("scripts,ipkg-build: use realpath for pkg_dir")
and there are assorted other uses of it during a build.
It is ordinarily provided by GNU coreutils. This adds a SetupHostCommand
to locate it either under its own name or under grealpath, the name that
it will be available as under MacPorts or Homebrew, which use
--program-prefix=g.

Similarly, update the SetupHostCommand for getopt to be able to locate a
util-linux getopt at the default path used by MacPorts, in the same
fashion that
commit cc16f5d73e77 ("build: try to find getopt in macOS homebrew's standard location")'
did for Homebrew. As there is no standard alternative --program-prefix
for util-linux utilities in the way that GNU packages often use a "g"
prefix, this path-based approach is required in case a non-util-linux
getopt (such as one provided by an OS) shadows the util-linux getopt
in the PATH.

Signed-off-by: Mark Mentovai <mark@moxienet.com>