Wrap defining of PARROT_GC_DEFAULT_TYPE under ifndef to make it overridable via compiler flags.
Wrap defining of PARROT_GC_DEFAULT_TYPE under ifndef to make it overridable
via compiler flags.

parrot/parrot,parrot/parrot,youprofit/parrot,youprofit/parrot,tewk/parrot-select,FROGGS/parrot,tewk/parrot-select,gitster/parrot,gitster/parrot,gagern/parrot,FROGGS/parrot,gitster/parrot,FROGGS/parrot,youprofit/parrot,tkob/parrot,FROGGS/parrot,youprofit/parrot,youprofit/parrot,gagern/parrot,FROGGS/parrot,tewk/parrot-select,gagern/parrot,parrot/parrot,tewk/parrot-select,gagern/parrot,parrot/parrot,gagern/parrot,gitster/parrot,parrot/parrot,gagern/parrot,tkob/parrot,tkob/parrot,gitster/parrot,gitster/parrot,youprofit/parrot,tkob/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,tewk/parrot-select,FROGGS/parrot,gitster/parrot,tkob/parrot,FROGGS/parrot,tkob/parrot,tkob/parrot,tewk/parrot-select,FROGGS/parrot,tewk/parrot-select,gagern/parrot