[C++] Error for missing endef

A fix for https://github.com/google/kati/issues/55
diff --git a/parser.cc b/parser.cc
index 96b8ba3..e88a9d9 100644
--- a/parser.cc
+++ b/parser.cc
@@ -90,6 +90,9 @@
 
     if (!if_stack_.empty())
       ERROR("%s:%d: *** missing `endif'.", loc_.filename, loc_.lineno + 1);
+    if (!define_name_.empty())
+      ERROR("%s:%d: *** missing `endef', unterminated `define'.",
+            loc_.filename, define_start_line_);
   }
 
   static void Init() {