Skip test for IPC::Run if user is overriding our search for PROVE.

The check for IPC::Run we added in commit c254970a is useful in simple
cases, but there are real use-cases where "prove" is coming from a
different Perl installation than the "perl" we want to use to build.
In such cases asking whether "perl" knows about IPC::Run is irrelevant
and can cause an unnecessary configure failure.  Hence, if user has
specified a value for PROVE, skip the IPC::Run check.  Per discussion
with Andrew Dunstan.

Discussion: https://postgr.es/m/E1dcE5n-0005Sk-UE@gemulon.postgresql.org