In makedef.pl, consolidate all the C compiler flags parsing code.

In makedef.pl, consolidate all the C compiler flags parsing code.

This isn't quite a "pure" refactoring, as -Dfoo=bar read from %Config will
now be processed as $Config{foo} = 'bar'; instead of $Config{foo} = 1;
However, it won't change any behaviour, as the only time makedef.pl uses a
value of %define for anything other than a truth test is $define{PERL_DLL},
and that is passed in on the command-line by the Makefile invoking
makedef.pl