Building tools with -fno-canonical-system-headers"

[Robert Elz <kre%munnari.OZ.AU@localhost>]

In order to test building pkgsrc/multimedia/ffmpeg3 with my latest sh mods
I needed to build a system with X11 included (normally I don't bother, X
doesn't affect how sh works) - this for the first time in ages.

When I enabled that I am seeing ...

dependall ===> tools/makekeys
#   compile  makekeys/makekeys.lo
cc -O -I/usr/obj/testing/tools/include/compat -I/release/testing/src/tools/compat -DHAVE_NBTOOL_CONFIG_H=1 -D_FILE_OFFSET_BITS=64 -I. -Wp,-fno-canonical-system-headers -DCSRG_BASED -DFUNCPROTO=15 -DNARROWPROTO -I/release/testing/amd64/usr/X11R7/include -D__AMD64__ -c -o makekeys.lo.o    /release/testing/src/xsrc/external/mit/libX11/dist/src/util/makekeys.c
cc1: error: unrecognized command line option "-fno-canonical-system-headers"

The host (build) system is a fairly old NetBSD (6 vintage) which is not
going to be upgraded any time soon.   (And yes, that means it is an ancient
sh which is being used for the building, none of my mods are relevant!)
On the host "man gcc" doesn't mention "no-canonical-system-headers: anywhere.
It is: gcc version 4.5.4 (NetBSD nb1 20120916)

Does anyone recognise what is going on there, and what is the right way
to overcome it?   Should we perhaps be building tools using the tools gcc?

The sources being built were updated just before the build, so apart
from any commits in the past hour should be up to date.

kre