I lived in that world, sunos, solaris, unicos, hpux, aix, irix, ultrix. I always found it preferable to have Os and even compiler specific includes for a whatever definitions needs to be different. and I certainly wasn't interested in having a rewrite pass on the makefiles. but it worked well enough when I dug up something on archie.

and then they wrapped it another layer of translation, and then another and I think more. and things started breaking because I needed the right version of autotools, and at some point I started just running "cc *.c -I. -o foo" before even starting to try to use configure, and it usually worked and saved me a lot of screwing around.

its certainly settled down some, but only because everything is really linux. but still we're here more than 30 years later checking for socklen_t.

it was a always a bad plan.

So what do you check for? Assuming things works because __linux__ == glibc + Linux leads to all sorts of fun