cflags.SH: Turn off attribute_nonnull warnings

gcc emits a bunch of warnings under -Wall for functions that have
declared that parameter 'n' is never going to be NULL, and then have the
temerity to make sure that that is true.

The problem is that we have many macros that are generalized enough to
handle the case where the parameter is NULL.  It just happens that
sometimes they get called as well from code where it is known to be not
NULL.  We could write additional macros that are known to take a
non-NULL parameter and don't do the test.  The existing macros would be
rewritten to just call the new ones after checking for non-NULL.

But that would complicate our code, and a main point of compilers doing
optimization is to figure out and remove impossible cases, without the
programmers having to be concerned with that.

Just turn off these warnings

Author: khwilliamson

cflags.SH

@@ -184,11 +184,20 @@ Intel*) ;; # # Is that you, Intel C++?
 # NOTE 3: the relative order of these options matters:
 # -Wextra before -Wno-long-long -Wno-declaration-after-statement
 #
+# NOTE 4: If a function has __attribute__nonnull__(n), gcc generates many
+#         warnings for macros that have the generality to check for NULLness on
+#         parameter n.  We could write versions of those macros that eliminate
+#         that check, but why?.  It complicates our code and gains no
+#         perceptible performance except in very hot code that the compiler
+#         has chosen to not optimize away.  So turn these off with
+#         -Wno-nonnull-compare
+
 *) warns="$pedantic \
 	-Werror=pointer-arith \
 	-Werror=vla \
 	-Wextra \
 	-Wno-long-long -Wno-declaration-after-statement \
+        -Wno-nonnull-compare \
 	-Wc++-compat -Wwrite-strings"
    case " $ccflags " in
    *" -std="*) ;; # Already have -std=...