I have a SUBDIRS project with 3 subprojects. Let's call then A, B, and C.

However, C is not actually a Qt project but is a pre-written makefile that needs to be called. What is the best syntax to integrate this into the SUBDIRS project?

Keep in mind the complications arise with shadow building. Just letting QMake "see" the Makefile already in the directory is going to break on a shadow build.