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

config/perl.m4

@@ -4,10 +4,7 @@
 # PGAC_PATH_PERL
 # --------------
 AC_DEFUN([PGAC_PATH_PERL],
-[# Let the user override the search
-if test -z "$PERL"; then
-  AC_PATH_PROG(PERL, perl)
-fi
+[PGAC_PATH_PROGS(PERL, perl)
 
 if test "$PERL"; then
   pgac_perl_version=`$PERL -v 2>/dev/null | sed -n ['s/This is perl.*v[a-z ]*\([0-9]\.[0-9][0-9.]*\).*$/\1/p']`