Building GCC, compiler flags do not propagate through Makefiles

[Gordon Steemson]

I'm attempting to build GCC 6.5.0 as an intermediate step to building newer things.  My present compiler is Apple's version of GCC 4.2.1.  I'm building in 64-bit mode on a dual-CPU Power Mac G5 from 2005.  I have run into the problem that I need to set the "-mlongcall" compiler flag because it otherwise errors out with a "branch is farther than 16M" complaint – but the Makefiles appear to explicitly prevent things like $CFLAGS from propagating to subsidiary Makefiles in subdirectories.  How do I get the blasted thing to accept the required flag all the way through to the last Makefile?

Gordon S.