patch 8.1.0991: cannot build with a mix of features

Problem:    Cannot build with FEAT_EVAL defined and FEAT_SEARCH_EXTRA
            undefined, and with FEAT_DIFF defined and FEAT_EVAL undefined.
Solution:   Add a couple of #ifdefs. (closes #4067)

src/diff.c

@@ -866,7 +866,11 @@ theend:
     int
 diff_internal(void)
 {
-    return (diff_flags & DIFF_INTERNAL) != 0 && *p_dex == NUL;
+    return (diff_flags & DIFF_INTERNAL) != 0
+#ifdef FEAT_EVAL
+	&& *p_dex == NUL
+#endif
+	;
 }
 
 /*

src/search.c

@@ -567,7 +567,9 @@ set_last_search_pat(
 	    saved_spats[idx].pat = NULL;
 	else
 	    saved_spats[idx].pat = vim_strsave(spats[idx].pat);
+# ifdef FEAT_SEARCH_EXTRA
 	saved_spats_last_idx = last_idx;
+# endif
     }
 # ifdef FEAT_SEARCH_EXTRA
     /* If 'hlsearch' set and search pat changed: need redraw. */

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    991,
 /**/
     990,
 /**/