The general way to do this is to create an m4 file with a 'case' in it
based on the "$host_os", e.g. (as taken and expanded from
grc_gr_audio_osx.m4; the case strings may or not be entirely correct,
and are used as examples):
-------------
passed=yes
[set some other global variables to expected values]
case "$host_os" in
darwin*)
[set some variable here for Darwin only]
;;
freebsd*)
[set some variable here for freebsd only]
;;
linux*)
[set some variable here for linux only]
;;
*)
[default, maybe print something?; set some variable?]
passed=no
;;
esac
[perform checks on the variables, to make sure they work together; set
"passed=no" on failure]
if test $passed = yes; then
AC_MSG_RESULT ([passed; using tap/tun FOO])
else
AC_MSG_RESULT ([failed; not using tap/tun])
fi
-------------
then one can change the "$host_os" case switch text to fit all needs.
As another example, it's possible that there is a difference in the way
OSX implements tap/tun between 10.4 and 10.5 ... in that case, there
would be different 'darwin8*' and 'darwin9*' cases. The same might hold
between Linux kernels 2.4.X and 2.6.X, and I'm sure there's a way to
create cases for each of those.