Use makedef.pl on VMS to feed vms/gen_shrfls.pl.

Use makedef.pl on VMS to feed vms/gen_shrfls.pl.

This replaces the latter's long-standing method of running
perl.h through the C preprocessor and parsing the output in
order to see what symbols need exporting in the linker options
file.

vms/gen_shrfls.pl remains part of the process for now since it
knows various things about generating linker options files on
different architectures and other VMS-specific gotchas such as
symbol case sensitivity and long symbol shortening.  These
features could be added to makedef.pl but are unlike anything
currently done there.

This should slightly increase the chances that folks can modify
the API without breaking the build, and it should make us a bit
stricter about only exporting the symbols we intend to, but the
result is still far from optimal.  It replaces one set of
heuristics and manually-maintained inclusion and exclusion lists
with different heuristics and even longer lists of inclusions and
exclusions.