Added NXT_MAYBE_UNUSED for __attribute__((__unused__)).

When testing some configurations of compilers and OSes, I noticed
that clang(1) 13 on Debian caused a function to be compiled but
unused, and the compiler triggered a compile error.

To avoid that error, use __attribute__((__unused__)).  Let's call
our wrapper NXT_MAYBE_UNUSED, since it describes itself more
precisely than the GCC attribute name.  It's also the name that
C2x (likely C23) has given to the standard attribute, which is
[[maybe_unused]], so it's also likely to be more readable because
of that name being in ISO C.

auto/clang

@@ -176,3 +176,21 @@ nxt_feature_test="struct s {
                       return 1;
                   }"
 . auto/feature
+
+
+nxt_feature="GCC __attribute__ unused"
+nxt_feature_name=NXT_HAVE_GCC_ATTRIBUTE_UNUSED
+nxt_feature_run=
+nxt_feature_incs=
+nxt_feature_libs=
+nxt_feature_test="static void f(void) __attribute__ ((__unused__));
+
+                  static void f(void)
+                  {
+                      return;
+                  }
+
+                  int main(void) {
+                      return 0;
+                  }"
+. auto/feature