Bug 1310: Set CTEST_CUSTOM_<XXX> variable only if @CTEST_CUSTOM_<XXX>@ is not empty. This solves the issue that compiler errors were no longer reported, because an empty string evaluated to integer value 0.

Bug 1310: Make all the CTEST_CUSTOM_* variables configurable using variants files. Added CTEST_CUSTOM_WARNING_EXCEPTION for PVSS software on kis001

Bug 1310: Generate CTestCustom.cmake file. This is needed, because settings like CTEST_CUSTOM_MAXIMUM_NUMBER_OF_WARNINGS need to be set in a CTestCustom.cmake file in the BINARY tree.