Makefile optimizations

[raboof]

Our makefiles contain ... = $(shell ...) and ... ?= $(shell ...) in several places, which are 'lazy', meaning the shell command is executed each time the variable is used.

We should consider replacing the lazy sets with immediate sets (:=), and for the 'lazy set if absent' cases consider whether it's worth adding additional logic.

[wilhelmy]

It's also possible to export the variables like in lua-detect.mk.