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/search.c b/src/search.c
index f62858c..94b1ef1 100644
--- a/src/search.c
+++ b/src/search.c
@@ -476,6 +476,7 @@
     return FALSE;
 }
 
+#if defined(FEAT_EVAL) || defined(PROTO)
     char_u *
 last_csearch(void)
 {
@@ -510,6 +511,7 @@
 	vim_memset(lastc_bytes, 0, sizeof(lastc_bytes));
 #endif
 }
+#endif
 
     void
 set_csearch_direction(int cdir)