Set OpenGL_GL_PREFERENCE to silence CMake CMP0072 warning

Geant4/G4config/CMakeLists.txt

@@ -38,6 +38,8 @@ endif()
 
 set(GEANT4_TAG ${GEANT4_TAG} CACHE STRING "Tag of Geant4 to get from the Geant4-srcs repository")
 
+set(OpenGL_GL_PREFERENCE "GLVND" CACHE STRING "Preferred GL library to use (see https://cmake.org/cmake/help/v3.12/policy/CMP0072.html)")
+
 include(ExternalProject)
 
 ExternalProject_Add(Geant4
@@ -64,6 +66,7 @@ ExternalProject_Add(Geant4
 #             -DLCG_USE_NATIVE_COMPILER=${LCG_USE_NATIVE_COMPILER}
               -DCMAKE_MODULE_PATH=${Geant4Files_DIR}
               -DGEANT4_INSTALL_EXAMPLES=OFF
+              -DOpenGL_GL_PREFERENCE=${OpenGL_GL_PREFERENCE}
 )
 
 # Hardcoded list of external packages used by Geant4, needed to prepare the