Add missing XERCESC_ROOT_DIR to cmake config

.gitlab-ci.yml

@@ -51,7 +51,7 @@ slc6-gcc62-XERCESC:
     - source .dd4hep-ci.d/init_x86_64.sh
     - mkdir build
     - cd build
-    - cmake -GNinja -D DD4HEP_USE_GEANT4=ON -D DD4HEP_USE_BOOST=ON -DBoost_NO_BOOST_CMAKE=ON -D DD4HEP_USE_LCIO=ON -D BUILD_TESTING=ON -D Geant4_DIR=$G4INSTALL/lib64/Geant4-10.2.2 -D DD4HEP_USE_CXX11=ON -DCMAKE_BUILD_TYPE=Release -DDD4HEP_USE_XERCESC=ON ..
+    - cmake -GNinja -D DD4HEP_USE_GEANT4=ON -D DD4HEP_USE_BOOST=ON -DBoost_NO_BOOST_CMAKE=ON -D DD4HEP_USE_LCIO=ON -D BUILD_TESTING=ON -D Geant4_DIR=$G4INSTALL/lib64/Geant4-10.2.2 -D DD4HEP_USE_CXX11=ON -DCMAKE_BUILD_TYPE=Release -DDD4HEP_USE_XERCESC=ON -DXERCESC_ROOT_DIR=${XercesC_HOME} ..
     - ninja
     - ninja install
     - . ../bin/thisdd4hep.sh