nedmalloc: work around overzealous GCC 6 warning

With GCC 6, the strdup() function is declared with the "nonnull"
attribute, stating that it is not allowed to pass a NULL value as
parameter.

In nedmalloc()'s reimplementation of strdup(), Postel's Law is heeded
and NULL parameters are handled gracefully. GCC 6 complains about that
now because it thinks that NULL cannot be passed to strdup() anyway.

Let's just shut up GCC >= 6 in that case and go on with our lives.

See https://gcc.gnu.org/gcc-6/porting_to.html for details.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/nedmalloc/nedmalloc.c

@@ -956,6 +956,9 @@ void **nedpindependent_comalloc(nedpool *p, size_t elems, size_t *sizes, void **
 char *strdup(const char *s1)
 {
 	char *s2 = 0;
+#if __GNUC__ >= 6
+#pragma GCC diagnostic ignored "-Wnonnull-compare"
+#endif
 	if (s1) {
 		size_t len = strlen(s1) + 1;
 		s2 = malloc(len);