BUILD: makefile: only compute alternative options if required

BUILD: makefile: only compute alternative options if required

Currently, the way the "cc-opt-alt" macro works consists in always
pre-calculating the alternative value for the case the main one would
not work, and pass both to an "if" clause in shell. Most of the time
we evaluate the second one for no reason.

Let's change this to use an internal "if" function instead, and directly
pass both option names to cc-opt-alt instead of passing a pre-calculated
expression. This saves one fork/exec per option and makes the option
easier to use.