Add warning if the authentication file not found and continue

Add warning if the authentication file not found and continue. The authentication file is not always needed. E.g. not needed for simple compilation checks.

Database/ConnectionManagement/AtlasAuthentication/CMakeLists.txt

@@ -36,8 +36,11 @@ endif()
 set( AtlasAuthentication_native_version "v18" )
 set( AtlasAuthentication_home
    "${ATLAS_EXTERNAL}/AtlasAuth/${AtlasAuthentication_native_version}" )
-atlas_install_xmls( ${AtlasAuthentication_home}/authentication.xml )
-
+if (EXISTS ${AtlasAuthentication_home}/authentication.xml )
+   atlas_install_xmls( ${AtlasAuthentication_home}/authentication.xml )
+else()
+   message(WARNING "The required authentication file ${AtlasAuthentication_home}/authentication.xml does not exist!")
+endif()
 # Configure the environment setup module:
 configure_file(
    ${CMAKE_CURRENT_SOURCE_DIR}/AtlasAuthenticationEnvironmentConfig.cmake.in