Variant: Prepare for C++17 support

Change-Id: I8f96ce3afae050d4e58e7fe9b70a92155045b71b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/variant.h

@@ -30,10 +30,25 @@
 */
 
 // TODO: replace by #include <(experimental/)variant> depending on compiler and C++ version
+#if __cplusplus >= 201703L
+#error Please delete variant.hpp and the #else section below, then remove this error
+#include <variant>
+
+namespace Utils {
+using std::get;
+using std::get_if;
+using std::holds_alternative;
+using std::variant;
+} // namespace Utils
+
+#else
 #include <3rdparty/variant/variant.hpp>
 
 namespace Utils {
-
+using mpark::get;
-using namespace mpark;
+using mpark::get_if;
-
+using mpark::holds_alternative;
+using mpark::variant;
 } // namespace Utils
+
+#endif