patch 8.1.0785: depending on the configuration some functions are unused

Problem:    Depending on the configuration some functions are unused.
Solution:   Add more #ifdefs, remove unused functions. (Dominique Pelle,
            closes #3822)
diff --git a/src/message.c b/src/message.c
index 0c3b532..1a7ec28 100644
--- a/src/message.c
+++ b/src/message.c
@@ -3507,12 +3507,14 @@
     --no_wait_return;
 }
 
+#if defined(FEAT_EVAL) || defined(PROTO)
     void
 give_warning2(char_u *message, char_u *a1, int hl)
 {
     vim_snprintf((char *)IObuff, IOSIZE, (char *)message, a1);
     give_warning(IObuff, hl);
 }
+#endif
 
 /*
  * Advance msg cursor to column "col".