For named subdirs, qmake creates a Makefile it subsequently ignores

The attached test project uses a named subdir project:

lib.file=lib/lib.pro
SUBDIRS=lib

(The naming is redundant here, but that's not relevant to the bug).

Upon invocation of qmake -r, a file called Makefile.lib is created in <build dir>/lib/. Then, when calling make, the following happens:

cd lib/ && ( test -f Makefile || /sda/home/christian/dev/qt5-stable-build/qtbase/bin/qmake /tmp/testproject/lib/lib.pro -o Makefile ) && make -f Makefile

So qmake does not seem to be entirely aware of its own naming convention.