autogen.sh: add --help description about the --with-distro switch

Change-Id: I8d9732251b05ec4ec93ce0fc0ba24f2f27d54b18
Reviewed-on: https://gerrit.libreoffice.org/69712
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

autogen.sh

@@ -203,6 +203,8 @@ for my $arg (@ARGV) {
     if ($arg =~ /^(--help|-h|-\?)$/) {
         print STDOUT "autogen.sh - libreoffice configuration helper\n";
         print STDOUT "   --clean        forcibly re-generate configuration\n";
+        print STDOUT "   --with-distro  use a config from distro-configs/\n";
+        print STDOUT "                  the name needs to be passed without extension\n";
         print STDOUT "   --best-effort  don't fail on un-known configure with/enable options\n";
         print STDOUT "\nOther arguments passed directly to configure:\n\n";
         system ("./configure --help");