Detect std::variant from CMake

CMakeLists.txt

@@ -497,6 +497,12 @@ else()
    endif()
 endif()
 
+try_compile( WALBERLA_USE_STD_VARIANT "${CMAKE_CURRENT_BINARY_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}/cmake/TestStdVariant.cpp"
+             COMPILE_DEFINITIONS -DWALBERLA_USE_STD_VARIANT ${CMAKE_CXX14_STANDARD_COMPILE_OPTION} )
+if( WALBERLA_USE_STD_VARIANT )
+   message( STATUS "Found std::variant")
+endif()
+
 
 
 ############################################################################################################################

cmake/TestStdVariant.cpp

+#include <iostream>
+#include <variant>
+
+int main() {
+   std::variant<int,float> a;
+   return 0;
+}

src/core/Variant.h

@@ -22,10 +22,6 @@
 #pragma once
 
 
-#if __cpp_lib_variant
-#define WALBERLA_USE_STD_VARIANT
-#endif
-
 #if defined(WALBERLA_USE_STD_VARIANT)
 #include <variant>
 #else