Disable the non-null compare warning/error.

We rely on the provided local library definitions for the hooked
functions which in some cases (GCC >6) carry a non-null-attribute flag
which causes compile errors on `!= NULL` checks.

src/Makefile

@@ -62,7 +62,7 @@ PREFIX ?= /usr/local
 LIBDIRNAME ?= /lib/faketime
 PLATFORM ?=$(shell uname)
 
-CFLAGS += -std=gnu99 -Wall -Wextra -Werror -DFAKE_STAT -DFAKE_SLEEP -DFAKE_TIMERS -DFAKE_INTERNAL_CALLS -fPIC -DPREFIX='"'$(PREFIX)'"' -DLIBDIRNAME='"'$(LIBDIRNAME)'"'
+CFLAGS += -std=gnu99 -Wall -Wextra -Werror -Wno-nonnull-compare -DFAKE_STAT -DFAKE_SLEEP -DFAKE_TIMERS -DFAKE_INTERNAL_CALLS -fPIC -DPREFIX='"'$(PREFIX)'"' -DLIBDIRNAME='"'$(LIBDIRNAME)'"'
 ifeq ($(PLATFORM),SunOS)
 CFLAGS += -D__EXTENSIONS__ -D_XOPEN_SOURCE=600
 endif