The recurse.mk file resides under $QNX_TARGET/usr/include/mk. This directory contains other files that are included within makefiles. Note that while the make utility automatically searches $QNX_TARGET/usr/include, we've created symbolic links from there to $QNX_TARGET/usr/include/mk.
The recurse.mk include file is typically used by higher-level makefiles to recurse into lower-level makefiles. All subdirectories present are scanned for files called makefile or Makefile. Any subdirectories that contain such files are recursed into, then make is invoked from within those directories, and so on, down the directory tree.
You can create a special file named Makefile.dnm (“dnm” stands for “Do Not Make”) next to a real Makefile to cause recurse.mk not to descend into that directory. The contents of Makefile.dnm aren't examined in any way—you can use touch to create an empty file for it.