With the integration of illumos#9027, the way to specify/disable C99 behavior in illumos makefiles changed. We should follow suit for our own illumos-joyent specific pieces.
Former user commented on 2018-05-21T11:37:58.022-0400:
Doing a simple search, this looks like the target list of things to change:
# find . -name Makefile\* | xargs grep C99MODE
./cmd/bhyve/test/Makefile.com:C99MODE= -xc99=%all
./cmd/varpd/Makefile:C99MODE= -xc99=%all
./cmd/vndadm/test/Makefile.com:C99MODE= -xc99=%all
./cmd/vndadm/Makefile:C99MODE= -xc99=%all
./lib/brand/lx/lx_brand/Makefile.com:C99MODE= -xc99=%all
./lib/libdladm/Makefile.com:C99MODE= -xc99=%all
./lib/libvnd/Makefile.com:C99MODE= -xc99=%all
./lib/varpd/svp/Makefile.com:C99MODE= -xc99=%all
./lib/varpd/direct/Makefile.com:C99MODE= -xc99=%all
./lib/varpd/files/Makefile.com:C99MODE= -xc99=%all
./lib/libctf/Makefile.shared.com:C99MODE= -xc99=%all