patch 7.4.2121
Problem:    No easy way to check if lambda and closure are supported.
Solution:   Add the +lambda feature.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 53783af..f665842 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5205,6 +5205,7 @@
 #ifdef FEAT_KEYMAP
 	"keymap",
 #endif
+	"lambda", /* always with FEAT_EVAL, since 7.4.2120 with closure */
 #ifdef FEAT_LANGMAP
 	"langmap",
 #endif