Line 36 of Makefile:
CXXFLAGS := $(CFLAGS) `fltk-config --cxxflags`
x86_64-pc-linux-gnu-g++ -I.. `fltk-config --cxxflags` -march=native -O2 -Wno-pointer-sign -DNUTDIR=\".nutdb\" -DFOODDIR=\"/usr/share/nut\" -DNOT_XFT -c -o Nut.o Nut.cc
cc1plus: warning: command line option '-Wno-pointer-sign' is valid for C/ObjC but not for C++ [enabled by default]
Oddly enough, even when Nut is built, we do not install it, and even then the DEPEND/RDEPEND on x11-libs/fltk is missing. I "fixed" it now (in 18.9) by not attempting to build Nut, and instead only build nut. If a users comes forward who wants Nut, that's for another time.
Fixing the actual bugs (mixing OPT/CFLAGS/CXXFLAGS randomly) in the Makefiles is probably something upstream should be informed of. The solution would be to use canonical variable names like INCLUDES.