Extend C++1z test to include variant APIs we use

std::get and std::visit are only available on macOS 10.14, so as long
as we're building with a deployment target lower than that, we can't
enable C++1z globally unless we special-case use of those functions.

Change-Id: Idb5eb5992ea4dd7eab92f5310321720e19ac793e
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

configure.json

@@ -283,7 +283,13 @@
                     "#else",
                     "#  error __cplusplus must be > 201402L (the value for C++14)",
                     "#endif",
-                    "#include <map>  // https://bugs.llvm.org//show_bug.cgi?id=33117"
+                    "#include <map>  // https://bugs.llvm.org//show_bug.cgi?id=33117",
+                    "#include <variant>"
+                ],
+                "main": [
+                    "std::variant<int> v(42);",
+                    "int i = std::get<int>(v);",
+                    "std::visit([](const auto &) { return 1; }, v);"
                 ],
                 "qmake": "CONFIG += c++11 c++14 c++1z"
             }