This years GCC rollover is nearing and xen fails to build on x86_64 with (first
error):

[  297s]   [BUILD] bin/null_sanboot.o
[  297s]   [BUILD] bin/stringextra.o
[  297s] core/stringextra.c: In function 'strtok':
[  297s] core/stringextra.c:189:18: error: nonnull argument 's' compared to
NULL [-Werror=nonnull-compare]
[  297s]   sbegin  = s ? s : ___strtok;
[  297s]             ~~~~~~^~~~~~~~~~~
[  297s]   [BUILD] bin/random.o
[  297s] cc1: all warnings being treated as errors
[  298s] bin/rules/core/stringextra.c.r:3: recipe for target
'bin/stringextra.o' failed
[  298s] make[7]: *** [bin/stringextra.o] Error 1
[  298s] make[7]: *** Waiting for unfinished jobs....

last year I added ipxe-no-error-logical-not-parentheses.patch that could be
amended to also add -Wno-error=nonnull-compare.

xen packaging is giving me head-aches (create patches to generate a patch file
and patch a series file?!), so while I can certainly hand-edit the existing
patch you for sure have scripts or a git repo where all this indirection is
hidden (at least I hope so for you ;)).

You can test building against GCC 6 via
--alternative-project=openSUSE:Factory:Staging:Gcc6