Build time configuration

Every more-than-trivial project needs configuration
The target environment may be X or Y
The host environment may be A or B
And some user choices may apply
Features to include or remove
Diagnostics to enable or disable

Build-time configuration is the source of countless bugs
Some configuration is rarely built
Some configuration is never built, actually
Un-polished code rusts, and then fails if you run it
Some combination is not even allowed

Please limit the number of options
Better, arrange for all options to be build-tested every time

This is why in fsmos I try to build all source files
The linker then discards what is not used
If your build takes minutes, or hours, please reconsider