Don't build VariantTest if Boost is not available

src/waLBerlaDefinitions.in.h

@@ -53,6 +53,7 @@
 #cmakedefine WALBERLA_USE_STD_ANY
 #cmakedefine WALBERLA_USE_STD_EXPERIMENTAL_OPTIONAL
 #cmakedefine WALBERLA_USE_STD_OPTIONAL
+#cmakedefine WALBERLA_USE_STD_VARIANT
 #cmakedefine WALBERLA_BUILD_WITH_BACKTRACE
 #ifdef WALBERLA_BUILD_WITH_BACKTRACE
 #define WALBERLA_BACKTRACE_HEADER "${Backtrace_HEADER}"

tests/core/CMakeLists.txt

@@ -209,8 +209,10 @@ waLBerla_execute_test( NAME UNIQUEID PROCESSES 4)
 waLBerla_compile_test( FILES VersionTest.cpp )
 waLBerla_execute_test( NAME VersionTest )
 
-waLBerla_compile_test( FILES VariantTest )
-waLBerla_execute_test( NAME VariantTest )
+if( WALBERLA_BUILD_WITH_BOOST OR WALBERLA_USE_STD_VARIANT )
+  waLBerla_compile_test( FILES VariantTest )
+  waLBerla_execute_test( NAME VariantTest )
+endif()
 
 ##################
 # load_balancing #