this is necessary for having a clean way to recurse even from leaf
makefiles.
the location where the variable is written in the makefile is somewhat
bizarre, but the code is so convoluted that finding a better place would
mean either a lot of duplication or a lot of prior refactoring.
Change-Id: I68e4cf7e3814f6c60b2e3421d69775c993dafb23
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
80b99a6dd7