Fix for ROOT-6861

in presence of a -D without any value, rootcling now automatically
sets the value to 1 in order to mimick what happens with a real compiler
invocation and a parsing of the payload.
Generate empty rootpcm files with rootcling/genreflex flag

rather than using an hardcoded list of root modules.
This has the advantage of being more flexible and to work
transparently for cmake and makefile.