build: Filter out --jobserver-auth from MAKEFLAGS

[ upstream commit cdecbcb ]
[ backporter's notes: manual conflict resolution ]

GNU make on the host may use --jobserver-style=fifo (default on my
machine). It also implies --jobserver-auth=fifo:/tmp/GMfifo$MAKE_PID,
an undocumented flag, used internally by make and passed to the child
instances of make. This flag appears in $(MAKEFLAGS).

The cilium-build target in Documentation/Makefile passes MAKEFLAGS to
another make instance, called in a docker image. The problem is that
--jobserver-auth passed to make inside docker points to a file that
doesn't exist in the container filesystem namespace, and make fails with
an error like this:

make: *** internal error: invalid --jobserver-auth string 'fifo:/tmp/GMfifo361142'.  Stop.
make: *** [Makefile:48: cilium-build] Error 2
make: Leaving directory '/home/max/.opt/go/src/github.com/cilium/cilium-snat/Documentation'

Fix this by filtering out --jobserver-auth=... from MAKEFLAGS when
passing it to make inside docker.

Signed-off-by: Maxim Mikityanskiy <[email protected]>

Documentation/Makefile

@@ -42,7 +42,7 @@ endif
 # documentation to be generated correctly.
 cilium-build:
 ifndef SKIP_BUILD
-	../contrib/scripts/builder.sh env MAKEFLAGS="$(MAKEFLAGS)" make build
+	../contrib/scripts/builder.sh env MAKEFLAGS="$(filter-out --jobserver-auth=%,$(MAKEFLAGS))" make build
 else
 	echo "SKIP_BUILD set, assuming all build artifacts are already present."
 endif