build: avoid -Wconditionally-supported on qsort check

It's OK to rely on conditionally-supported features in #if CHECKING_P, since
that isn't defined in stage 1.

gcc/ChangeLog:

	* sort.cc: Disable -Wconditionally-supported in
	CHECKING_P code.

gcc/sort.cc

@@ -237,6 +237,10 @@ do {                                            \
 }
 
 #if CHECKING_P
+  /* Don't complain about cast from void* to function pointer.  */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wconditionally-supported"
+
 /* Adapter for using two-argument comparators in functions expecting the
    three-argument sort_r_cmp_fn type.  */
 static int
@@ -266,6 +270,7 @@ gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
     free (buf);
 #if CHECKING_P
   qsort_chk (vbase, n, size, cmp2to3, (void*)cmp);
+#pragma GCC diagnostic pop
 #endif
 }