Running make when all the BUILD_* vars are set to 0 causes make to repeatedly invoke itself until interrupted by ctrl-c, outputting something like

make[15]: *** [targets] Interrupt
make[14]: *** [release] Interrupt
make[13]: *** [targets] Interrupt
[...]

when you do so.
This might sound a bit like a case of "well don't do that then", but:
1) imo it should be able to detect this condition and die gracefully instead of exhibiting the somewhat confusing behaviour described
2) if you attempt to run make-macosx-ub.sh with BUILD_SERVER=0 in Makefile.local (which isn't unreasonable to do), the script disables all the other targets while attempting to build the dedicated server and triggers the bug. This could be fixed by explicitly enabling the server target in the script (interestingly, this bug is not triggered by BUILD_CLIENT=0, although arguably the error given still isn't ideal).