Porting apcupsd seem to be quite easy at least at first glance.

OS detection:
it is done configure.in with the macro AM_CONDITIONAL and AC_DEFINE.
The AM_CONDITIONAL is for Makefile generating HAVE_SUN_OS_TRUE=' ' or
HAVE_SUN_OS_TRUE='#' for example if you have or not SUNOS. It expand these
variables and they become comments or spaces. Now it is used for selecting the
compiler switches.
The AC_DEFINE define HAVE_SUN_OS in the config.h file so that the C
preprocessor can know which OS we are compiling for.
For adding a new OS:
In configure.in add:

if test $HAVE_UNAME=yes -a x`uname -s` = xSunOS
then
        AM_CONDITIONAL(HAVE_NEW_OS, /bin/true)
        AC_DEFINE(HAVE_NEW_OS)
else
        AM_CONDITIONAL(HAVE_NEW_OS, /bin/false)
fi

In acconfig.h add:

#undef HAVE_NEW_OS

then regenerate the configure script and config.h.in files with autoregen.sh.

This thing is useful for compilers too. I have added checks for GCC so that
the Makefile know if we are using GCC or not.