YES!!! I finally found the problem! Turns out gnustep-base installs a make file, base.make, under $GNUSTEP_MAKEFILES/Additional during the make install process, as opposed to installing the file under $GNUSTEP_INSTALLATION_DIR/Library/Makefiles/Additional (which is where it is supposed to go). So whenever I make the package, it's created without this file, which apparently is where
NXConstStr.h is defined for the system. Moving the file there manually fixes everything!