system/lxdm: Allow building against gtk+3.

Signed-off-by: Matteo Bernardini <ponce@slackbuilds.org>

system/lxdm/README

@@ -2,4 +2,7 @@ LXDM - GUI login manager for LXDE
 
 LXDM is a lightweight drop-in replacement for GDM or KDM.
 
+To build the GTK3 version, script can be run as:
+  GTK3=yes ./lxdm.SlackBuild
+
 See README.SLACKWARE for post-installation configuration.

system/lxdm/lxdm.SlackBuild

@@ -3,7 +3,7 @@
 # Slackware build script for lxdm
 
 # Copyright 2010,2011  Robby Workman, Northport, Alabama, USA
-# Copyright 2012-2015  Matteo Bernardini, Pisa, Italy
+# Copyright 2012-2017  Matteo Bernardini, Pisa, Italy
 # All rights reserved.
 #
 # Redistribution and use of this script, with or without modification, is
@@ -25,7 +25,7 @@
 
 PRGNAM=lxdm
 VERSION=${VERSION:-0.5.3}
-BUILD=${BUILD:-2}
+BUILD=${BUILD:-3}
 TAG=${TAG:-_SBo}
 
 if [ -z "$ARCH" ]; then
@@ -55,6 +55,8 @@ else
   LIBDIRSUFFIX=""
 fi
 
+with_gtk3="" ; [ "${GTK3:-no}" != "no" ] && with_gtk3="--enable-gtk3"
+
 set -e
 
 rm -rf $PKG
@@ -101,6 +103,7 @@ CXXFLAGS="$SLKCFLAGS" \
   --mandir=/usr/man \
   --docdir=/usr/doc/$PRGNAM-$VERSION \
   --without-pam \
+  $with_gtk3 \
   --build=$ARCH-slackware-linux
 
 make