Warning:
root/core/rootcling_stage1/src/rootcling_stage1.cxx(38):
	warning #69: integer conversion resulted in truncation

   auto dummyVal = (int)(long)&usedToIdentifyRootClingByDlSym;
                    ^^^

The method above uses a cast to long, followed by a cast to int,
which results in a truncation. That is harmless, since the value
is never used, but generates a compiler warning with ICC 17. This
commit avoids the warning by storing the address of the same symbol
in a static variable instead.