Improve MBString backfill (addresses #755)
Improve MBString backfill (addresses #755)

Instead of checking for the existence of a particular function we
now check for the presence of the MBString extension. When defining
the MBString constants use the def() function. Rather than checking
for the presense of a single MBString function check every function
prior to creation.

The above changes were made in response to an incomplete MBString
backfill performed by WordPress.

BrandoCommando/dompdf,VictoriaLa/dompdf,ecomstyle/dompdf,mubassirhayat/dompdf,BrunoDeBarros/dompdf,Shnoulle/dompdf,dompdf/dompdf,koenpunt/dompdf,BrandoCommando/dompdf,sunkanmi-akintoye/dompdf,marciomrs4/dompdf,IvoPereira/dompdf,IvoPereira/dompdf,sunkanmi-akintoye/dompdf,spanichev/dompdf,scotteh/dompdf,marijnvdwerf/dompdf,eluhr/dompdf,eluhr/dompdf,IvoPereira/dompdf,koenpunt/dompdf,sunfun/dompdf,sunfun/dompdf,lucas-issa/dompdf,VictoriaLa/dompdf,nathanntg/dompdf,edge-soft/dompdf,marijnvdwerf/dompdf,marciomrs4/dompdf,itzmukeshy7/dompdf,BrandoCommando/dompdf,rdpascua/dompdf,rdpascua/dompdf,VictoriaLa/dompdf,Shnoulle/dompdf,bcremer/dompdf,Soullivaneuh/dompdf,Magoja/dompdf,Shnoulle/dompdf,itzmukeshy7/dompdf,edge-soft/dompdf,Magoja/dompdf,theodorDiaconu/dompdf,chielsen/dompdf,Magoja/dompdf,theodorDiaconu/dompdf,bcremer/dompdf,ecomstyle/dompdf,lucas-issa/dompdf,lordgnu/dompdf,dannizhang/dompdf,theodorDiaconu/dompdf,mubassirhayat/dompdf,ecomstyle/dompdf,sunkanmi-akintoye/dompdf,marciomrs4/dompdf,scotteh/dompdf,BrunoDeBarros/dompdf,craigfrancis/dompdf,Soullivaneuh/dompdf,lordgnu/dompdf,lucas-issa/dompdf,spanichev/dompdf,rdpascua/dompdf,nathanntg/dompdf,dannizhang/dompdf,marijnvdwerf/dompdf,bcremer/dompdf,dompdf/dompdf,eluhr/dompdf,chielsen/dompdf,craigfrancis/dompdf,chielsen/dompdf,mubassirhayat/dompdf,koenpunt/dompdf,dannizhang/dompdf,Soullivaneuh/dompdf,spanichev/dompdf,itzmukeshy7/dompdf,craigfrancis/dompdf,sunfun/dompdf