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/docbook.m4

@@ -3,7 +3,7 @@
 # PGAC_PROG_NSGMLS
 # ----------------
 AC_DEFUN([PGAC_PROG_NSGMLS],
-[AC_PATH_PROGS([NSGMLS], [onsgmls nsgmls])])
+[PGAC_PATH_PROGS(NSGMLS, [onsgmls nsgmls])])
 
 
 # PGAC_CHECK_DOCBOOK(VERSION)

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']`

config/programs.m4

@@ -1,6 +1,24 @@
 # config/programs.m4
 
 
+# PGAC_PATH_PROGS
+# ---------------
+# This wrapper for AC_PATH_PROGS behaves like that macro except when
+# VARIABLE is already set; in that case we just accept the value verbatim.
+# (AC_PATH_PROGS would accept it only if it looks like an absolute path.)
+# A desirable future improvement would be to convert a non-absolute-path
+# input into absolute form.
+AC_DEFUN([PGAC_PATH_PROGS],
+[if test -z "$$1"; then
+  AC_PATH_PROGS($@)
+else
+  # Report the value of $1 in configure's output in all cases.
+  AC_MSG_CHECKING([for $1])
+  AC_MSG_RESULT([$$1])
+fi
+])
+
+
 # PGAC_PATH_BISON
 # ---------------
 # Look for Bison, set the output variable BISON to its path if found.
@@ -8,10 +26,7 @@
 # Note we do not accept other implementations of yacc.
 
 AC_DEFUN([PGAC_PATH_BISON],
-[# Let the user override the search
-if test -z "$BISON"; then
-  AC_PATH_PROGS(BISON, bison)
-fi
+[PGAC_PATH_PROGS(BISON, bison)
 
 if test "$BISON"; then
   pgac_bison_version=`$BISON --version 2>/dev/null | sed q`
@@ -41,7 +56,7 @@ if test -z "$BISON"; then
 *** PostgreSQL then you do not need to worry about this, because the Bison
 *** output is pre-generated.)])
 fi
-# We don't need AC_SUBST(BISON) because AC_PATH_PROG did it
+# We don't need AC_SUBST(BISON) because PGAC_PATH_PROGS did it
 AC_SUBST(BISONFLAGS)
 ])# PGAC_PATH_BISON
 
@@ -229,7 +244,7 @@ AC_DEFUN([PGAC_CHECK_GETTEXT],
                  [AC_MSG_ERROR([a gettext implementation is required for NLS])])
   AC_CHECK_HEADER([libintl.h], [],
                   [AC_MSG_ERROR([header file <libintl.h> is required for NLS])])
-  AC_PATH_PROGS(MSGFMT, msgfmt)
+  PGAC_PATH_PROGS(MSGFMT, msgfmt)
   if test -z "$MSGFMT"; then
     AC_MSG_ERROR([msgfmt is required for NLS])
   fi
@@ -238,8 +253,8 @@ AC_DEFUN([PGAC_CHECK_GETTEXT],
     pgac_cv_msgfmt_flags=-c
 fi])
   AC_SUBST(MSGFMT_FLAGS, $pgac_cv_msgfmt_flags)
-  AC_PATH_PROGS(MSGMERGE, msgmerge)
-  AC_PATH_PROGS(XGETTEXT, xgettext)
+  PGAC_PATH_PROGS(MSGMERGE, msgmerge)
+  PGAC_PATH_PROGS(XGETTEXT, xgettext)
 ])# PGAC_CHECK_GETTEXT
 
 

config/python.m4

@@ -6,10 +6,10 @@
 
 # PGAC_PATH_PYTHON
 # ----------------
-# Look for Python and set the output variable 'PYTHON'
-# to 'python' if found, empty otherwise.
+# Look for Python and set the output variable 'PYTHON' if found,
+# fail otherwise.
 AC_DEFUN([PGAC_PATH_PYTHON],
-[AC_PATH_PROG(PYTHON, python)
+[PGAC_PATH_PROGS(PYTHON, python)
 if test x"$PYTHON" = x""; then
   AC_MSG_ERROR([Python not found])
 fi

config/tcl.m4

@@ -4,7 +4,7 @@
 
 
 AC_DEFUN([PGAC_PATH_TCLSH],
-[AC_PATH_PROGS(TCLSH, [tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84])
+[PGAC_PATH_PROGS(TCLSH, [tclsh tcl tclsh8.6 tclsh86 tclsh8.5 tclsh85 tclsh8.4 tclsh84])
 if test x"$TCLSH" = x""; then
   AC_MSG_ERROR([Tcl shell not found])
 fi