The CXX_STANDARD property validates the values allowed for it, and '90' is not a valid value for it. However, the validation is only done for languages enabled for the target, and as C_undefined has no CXX files, that language is not enabled for it. This amends commit v3.1.0-rc3~23^2 (WCDH: Make the header -Wundef safe for the C language., 2014-11-24).