build: move Makefile checks to configure

[jbergstroem]

There's a few things checked in Makefile that could be moved to configure so it's not executed every time Makefile is sourced (which seems to happen multiple times while building with gmake).

If others think this is a good idea I'll whip a patch for a review.

[bnoordhuis]

What things are you thinking of?

[jbergstroem]

Just had a run through and it actually isn't being sourced as often as I initially believed. Let's just close this and I'll just file PR's with minor improvements as I go along instead?

[bnoordhuis]

Sounds like a plan. Thanks for following up, Johan.