allow PERL_DISABLE_PMC to be defined from anywhere

I enable PERL_DISABLE_PMC in a .h, not in CCFLAGS. CCFLAGS on Unix
typically has no -Ds in it, build options go into config.h, not
Config.pm's CCFLAGS. This change allows PERL_DISABLE_PMC option to pass
tests regardless of where it is defined. cpan/parent/t/parent-pmc.t is not
fixed by this patch.