Invalid Makefile generated for the subdirs template when using debug_and_release

When generating a Makefile for the subdirs template and using debug_and_release in CONFIG the Makefile generated will have an invalid dependency for sub-<directory>-sub_Debug, for instance when "dir1" is one of the subdirectories then the Makefile will contain the following lines

dir1/$(MAKEFILE):
@$(CHK_DIR_EXISTS) dir1/ || $(MKDIR) dir1/
cd dir1/ && $(QMAKE) dir1.pro -unix CONFIG+=thread -o $(MAKEFILE)
[...]
sub-dir1-sub_Debug: dir1//$(MAKEFILE)
cd dir1/ && $(MAKE) debug

and the double slash in the second last line above will cause make to fail.