[gimp] configure: --enable-check-update now with an "auto" value.

[Jehan <jehanp src gnome org>]

commit 0e46fadf439988ddc56a59133d55c5a6ec019058
Author: Jehan <jehan girinstud io>
Date:   Thu Aug 26 01:19:58 2021 +0200

    configure: --enable-check-update now with an "auto" value.
    
    By default, when on "auto", the update check feature will be set
    depending on the OS. In particular, on Windows and macOS, it will be ON,
    because these are the 2 OSes which we distribute without an update
    channel, hence where people used to have very outdated versions of GIMP.
    
    On Linux, *BSD, and so on, distribution provide updates through package
    repositories.

 configure.ac | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 2fb9dffb2f..92b5e6bfd6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2655,11 +2655,19 @@ GIMP_PACKAGE_REVISION=$with_revision
 AC_SUBST(GIMP_PACKAGE_REVISION)
 
 AC_ARG_ENABLE(check-update,
-              [  --disable-check-update
-                          GIMP will not look up new version availability on startup],
-              check_update=no,
-              check_update=yes)
-if test "x$check_update" = xyes; then
+              [  --enable-check-update
+                          GIMP will not look up new version availability on startup (default=auto)],,
+              enable_check_update=auto)
+if test "x$enable_check_update" != xno &&
+   test "x$enable_check_update" != xyes; then
+  if test "x$platform_osx" = "xyes" ||
+     test "x$platform_win32" = "xyes"; then
+     enable_check_update=yes
+  else
+     enable_check_update=no
+  fi
+fi
+if test "x$enable_check_update" = xyes; then
   AC_DEFINE(CHECK_UPDATE, 1,
               [Define to 1 if automatic update check is enabled])
 fi
@@ -3189,7 +3197,7 @@ Extra Binaries:
   gimp-console:              $enable_gimp_console
 
 Optional Features:
-  Check updates at startup:  $check_update
+  Check updates at startup:  $enable_check_update
   Levin matting engine:      $have_matting_levin
   Language selection:        $have_iso_codes
   Vector icons:              $enable_vector_icons