Factor out all autosetup-processed @if/@else blocks from Makefile.in in prep for...

Factor out all autosetup-processed @if/@else blocks from Makefile.in in prep for moving most of the makefile code into main.mk (which has, so far, been completely overlooked in this port but will now become the main basis for the static parts of the build). The idea is that all build configuration goes into a platform-dependent makefile which then includes main.mk.

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