configure: More use of AC_ARG_VAR

AC_ARG_VAR is necessary if an environment variable influences a
configure result that is then used by other tests that are cached.
With AC_ARG_VAR, a change in the variable is detected on subsequent
configure runs and the user is then advised to remove the cache.

This adds AC_ARG_VAR calls for: MSGFMT, PERL, PYTHON, TCLSH, XML2_CONFIG

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/30672.1546816567@sss.pgh.pa.us

config/perl.m4

@@ -5,6 +5,7 @@
 # --------------
 AC_DEFUN([PGAC_PATH_PERL],
 [PGAC_PATH_PROGS(PERL, perl)
+AC_ARG_VAR(PERL, [Perl program])dnl
 
 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

@@ -245,6 +245,7 @@ AC_DEFUN([PGAC_CHECK_GETTEXT],
   AC_CHECK_HEADER([libintl.h], [],
                   [AC_MSG_ERROR([header file <libintl.h> is required for NLS])])
   PGAC_PATH_PROGS(MSGFMT, msgfmt)
+  AC_ARG_VAR(MSGFMT, [msgfmt program for NLS])dnl
   if test -z "$MSGFMT"; then
     AC_MSG_ERROR([msgfmt is required for NLS])
   fi

config/python.m4

@@ -17,6 +17,7 @@
 # newer version.
 AC_DEFUN([PGAC_PATH_PYTHON],
 [PGAC_PATH_PROGS(PYTHON, [python python3 python2])
+AC_ARG_VAR(PYTHON, [Python program])dnl
 if test x"$PYTHON" = x""; then
   AC_MSG_ERROR([Python not found])
 fi

config/tcl.m4

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

configure

@@ -887,8 +887,13 @@ PKG_CONFIG_PATH
 PKG_CONFIG_LIBDIR
 ICU_CFLAGS
 ICU_LIBS
+XML2_CONFIG
 LDFLAGS_EX
-LDFLAGS_SL'
+LDFLAGS_SL
+PERL
+PYTHON
+MSGFMT
+TCLSH'
 # Initialize some variables set by options.
@@ -1586,8 +1591,13 @@ Some influential environment variables:
               path overriding pkg-config's built-in search path
   ICU_CFLAGS  C compiler flags for ICU, overriding pkg-config
   ICU_LIBS    linker flags for ICU, overriding pkg-config
+  XML2_CONFIG path to xml2-config utility
   LDFLAGS_EX  extra linker flags for linking executables only
   LDFLAGS_SL  extra linker flags for linking shared libraries only
+  PERL        Perl program
+  PYTHON      Python program
+  MSGFMT      msgfmt program for NLS
+  TCLSH       Tcl interpreter program (tclsh)
 Use these variables to override the choices made by `configure' or to help
 it to find libraries and programs with nonstandard names/locations.

configure.in

@@ -917,6 +917,7 @@ PGAC_ARG_BOOL(with, libxml, no, [build with XML support],
 
 if test "$with_libxml" = yes ; then
   PGAC_PATH_PROGS(XML2_CONFIG, xml2-config)
+  AC_ARG_VAR(XML2_CONFIG, [path to xml2-config utility])dnl
   if test -n "$XML2_CONFIG"; then
     for pgac_option in `$XML2_CONFIG --cflags`; do
       case $pgac_option in