Migrate 'master' to G4 v10r3p1

This MR updates master to pickup G4 v10r3p1 sources from Geant4-srcs. It should be picked up in the nightlies to verify that everything compiles and works ok.

Geant4/G4config/CMakeLists.txt

@@ -38,13 +38,20 @@ endif()
 
 set(GEANT4_TAG ${GEANT4_TAG} CACHE STRING "Tag of Geant4 to get from the Geant4-srcs repository")
 
+# Fix the C++ standard for GCC 4.9
+if(${GAUDI_CXX_STANDARD} STREQUAL "c++1y" AND $ENV{CMTOPT} MATCHES "gcc49")
+  set(CXX_STANDARD "c++14")
+else()
+  set(CXX_STANDARD ${GAUDI_CXX_STANDARD})
+endif()
+
 include(ExternalProject)
 
 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