build: avoid make jobserver warnings

build: avoid make jobserver warnings

I observe:

	» make -j8 CCOPTS=-ggdb3
	lib
	make[1]: warning: -j8 forced in submake: resetting jobserver mode.
	make[1]: Nothing to be done for 'all'.
	ip
	make[1]: warning: -j8 forced in submake: resetting jobserver mode.
	    CC       ipntable.o

MFLAGS is a historic variable of some kind; removing it fixes the
jobserver issue.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>