discover.exe can't parse modversion `18beta1`

[rwmjones]

Downstream bug: https://bugzilla.redhat.com/show_bug.cgi?id=2385359

Configuration fails at:

Running[17]: (cd _build/default/lib && config/discover.exe)
File "lib/dune", lines 16-19, characters 0-91:
16 | (rule
17 |  (targets c_flags.sexp c_library_flags.sexp)
18 |  (action
19 |   (run ./config/discover.exe)))
Command [17] exited with code 1:
$ (cd _build/default/lib && config/discover.exe)
Unable to parse libpq version: 18beta1

I didn't look at this very closely, but it seems to happen because of the "unusual" modversion of the latest Fedora package:

$ pkgconf --modversion libpq
18beta1

which I guess the discover script doesn't like.