Right. I have a 250-line utility that is currently testing for FreeBSD, OpenBSD and Darwin in the Makefile (due to differences in which libraries need to be linked to get a function, and differences in the include and library paths), and tests for FreeBSD, OpenBSD, NetBSD and Darwin in the .c file (due to differences in which header file functions are declared).

If someone tells me it doesn't work on, say, Solaris, my next step will be to convert it to autoconf. The essential complexity is still there.