Fixed cxx standard: actually use the fixed value

Geant4/G4config/CMakeLists.txt

@@ -51,7 +51,7 @@ ExternalProject_Add(Geant4
    GIT_REPOSITORY ${GEANT4_SRCS_REPOSITORY}
    GIT_TAG ${GEANT4_TAG}
    CMAKE_ARGS -DCMAKE_TOOLCHAIN_FILE=${CMAKE_SOURCE_DIR}/toolchain.cmake
-              -DGEANT4_BUILD_CXXSTD=${GAUDI_CXX_STANDARD}
+              -DGEANT4_BUILD_CXXSTD=${CXX_STANDARD}
               -DGEANT4_INSTALL_DATA=OFF
               -DGEANT4_USE_SYSTEM_CLHEP=ON
               -DGEANT4_USE_XM=ON