Log Message:
lang/coq: update to 8.20.1

pkgsrc changes: g/c obsolete patches; update build mechanism.

 *** Note:
 ***
 *** Because of upstream build changes, the stdlib files have moved:
 ***    ${PREFIX}/lib/coq/theories
 ***  ==>
 ***    ${PREFIX}/lib/ocaml/site-lib/coq/theories
 ***
 *** This package includes a symlink in the old location; future
 *** versions might not.

Upstream changes since 8.15.2:
...way too many to paste here since it's been way too long since the
last update.

See https://coq.inria.fr/doc/v8.20/refman/changes.html.