pgsql: Further improve consistency of configure's program searching.

Further improve consistency of configure's program searching.

Peter Eisentraut noted that commit 40b9f1921 had broken a configure
behavior that some people might rely on: AC_CHECK_PROGS(FOO,...) will
allow the search to be overridden by specifying a value for FOO on
configure's command line or in its environment, but AC_PATH_PROGS(FOO,...)
accepts such an override only if it's an absolute path. We had worked
around that behavior for some, but not all, of the pre-existing uses
of AC_PATH_PROGS by just skipping the macro altogether when FOO is
already set. Let's standardize on that workaround for all uses of
AC_PATH_PROGS, new and pre-existing, by wrapping AC_PATH_PROGS in a
new macro PGAC_PATH_PROGS. While at it, fix a deficiency of the old
workaround code by making sure we report the setting to configure's log.

Eventually I'd like to improve PGAC_PATH_PROGS so that it converts
non-absolute override inputs to absolute form, eg "PYTHON=python3"
becomes, say, PYTHON = /usr/bin/python3. But that will take some
nontrivial coding so it doesn't seem like a thing to do in late beta.

Discussion: https://postgr.es/m/90a92a7d-938e-507a-3bd7-ecd2b4004689@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b21c569cea58a1396d9ffd8a7e7a84aa991a1123

Modified Files
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config/docbook.m4 | 2 +-
config/perl.m4 | 5 +-
config/programs.m4 | 31 +++++--
config/python.m4 | 6 +-
config/tcl.m4 | 2 +-
configure | 247 ++++++++++++++++++++++++++++++++++++++++++++++++-----
configure.in | 26 +++---
7 files changed, 267 insertions(+), 52 deletions(-)