Mksymlists: dont produce Borland C export symbols if BCC support dropped

I am unable to do a "make test" on my patch in EUMM repo because EUMM github master fails very badly, so it has only been tested in my P5P blead repo.

Borland C was dropped in Perl 5.16. Underscore prefixed symbols were added
for interworking between BCC DLLs and VC DLLs in P5P ML post
"[PATCH] binary coexistence on win32" by Gurusamy Sarathy. Remove
underscore prefixed symbols if BCC support is impossible on a particular
perl version. This removed a RO litteral string, plus some void * sized
slices in the export table, from every DLL image making them smaller.