Sounds like a bit of tool support would be useful. Note that the UNIX command patch has a -D <symbol> option, which results in the patch only being applied if

Sounds like a bit of tool support would be useful. Note that the UNIX command patch has a -D <symbol> option, which results in the patch only being applied if the (C/C++) program is compiled with -D <symbol>, so this idea is not actually all that new...