Add support for "else if" in gyp conditions

test/conditions/elseif/elseif_conditions.gypi

Index: test/conditions/elseif/elseif_conditions.gypi
diff --git a/test/conditions/elseif/elseif_conditions.gypi b/test/conditions/elseif/elseif_conditions.gypi
new file mode 100644
index 0000000000000000000000000000000000000000..4310ccc0312890c05b7f31f85d83e35f766e8b6f
--- /dev/null
+++ b/test/conditions/elseif/elseif_conditions.gypi
@@ -0,0 +1,15 @@
+{
+  'conditions': [
+    ['test_var==0', {
+      'defines': ['FOO="first_if"'],
+    }, 'test_var==1', {
+      'defines': ['FOO="first_else_if"'],
+    }, 'test_var==2', {
+      'defines': ['FOO="second_else_if"'],
+    }, 'test_var==3', {
+      'defines': ['FOO="third_else_if"'],
+    }, {
+      'defines': ['FOO="last_else"'],
+    }],
+  ],
+}