Add --enable-gp2x switch for gp2x-specific code.

configure.ac

@@ -104,6 +104,18 @@ fi
 
 AM_CONDITIONAL([LOWMEM], [test "x$lowmem" = "xyes"])
 
+AC_ARG_ENABLE([gp2x],
+              AS_HELP_STRING([--enable-gp2x], [enable support for GP2X Personal Entertainment Player]),
+	      [gp2x=$enableval],
+	      [gp2x=no])
+
+if test "x$gp2x" = "xyes"
+then
+	CPPFLAGS="$CPPFLAGS -DGP2X"
+fi
+
+AM_CONDITIONAL([GP2X], [test "x$gp2x" = "xyes"])
+
 DATADIR=$PACKAGE
 AC_ARG_WITH([datadir-name],
             AS_HELP_STRING([--with-datadir-name@<:@=DIR@:>@], [change name of data directory @<:@wesnoth@:>@]),