g_str_equal: Provide macro for optimization

g_str_equal() is a nicer API than strcmp()==0, and less error prone.
However, forcing a function call prevents compiler from doing
optimizations. In the case it is not used as callback to GHashTable,
provide a macro that calls strcmp directly. This also has the side
effect that it forces arguments to be `const char *` instead of
`gconstpointer` in the case it is not used as callback, which adds type
safety.

Fixes: #2775

glib/ghash.c

@@ -2323,8 +2323,8 @@ g_hash_table_get_values (GHashTable *hash_table)
  * Returns: %TRUE if the two keys match
  */
 gboolean
-g_str_equal (gconstpointer v1,
-             gconstpointer v2)
+(g_str_equal) (gconstpointer v1,
+               gconstpointer v2)
 {
   const gchar *string1 = v1;
   const gchar *string2 = v2;