How can I write a makefile to auto-detect and parallelize the build with GNU Make?

Question

Not sure if this is possible in one Makefile alone, but I was hoping to write a Makefile in a way such that trying to build any target in the file auto-magically detects the num

Answer 1

I'll skip over the $(NPROCS) detection stuff, but here's how you could do this in a single Makefile (this is probably GNU Make specific, but that looks like the variant you're running):

ifeq ($(NPROCS),)
# Code to set NPROCS...
%:
    $(MAKE) -j$(NPROCS) NPROCS=$(NPROCS)
else
# All of your targets...
endif

See Defining Last-Resort Default Rules and Overriding Part of Another Makefile in the GNU Make Manual.