Add support for "else if" in gyp conditions

test/conditions/elseif/program.cc

Index: test/conditions/elseif/program.cc
diff --git a/test/win/idl-excluded/program.cc b/test/conditions/elseif/program.cc
similarity index 81%
copy from test/win/idl-excluded/program.cc
copy to test/conditions/elseif/program.cc
index 9dc3c94f3431be00adb755f6515b995d734e089b..147fe2f75e46943b49f33420133adbb610d1e09d 100644
--- a/test/win/idl-excluded/program.cc
+++ b/test/conditions/elseif/program.cc
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <stdio.h>
+
 int main() {
+  printf("%s\n", FOO);
   return 0;
 }