pgsql: Skip test for IPC::Run if user is overriding our search for PROV

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

The check for IPC::Run we added in commit c254970ad 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8014d2afa7b817c98544cace7efc337ee891aa57

Modified Files
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configure | 123 ++++++++++++++++++++++++++++++-----------------------------
configure.in | 15 +++++---
2 files changed, 72 insertions(+), 66 deletions(-)