It already was in the build system. The macros that control conditional compilation when one is doing it with the preprocessor do not appear out of thin air. A build system has to set them appropriately in the first place.

All of the mechanism for auto-detecting (or manually specifying) the capabilities of the target have to be there. It is simply a case of whether they set preprocessor macros via command-line options or tell the build system to use alternative source files.

The latter has the advantage of only creating build dependencies against the modules that actually use the library functionality in question, rather than against the compiler command line and potentially forcing a re-build of everything (when an honest dependencies mechanism is being used).