gis/mapnik: Replaced backtick cmdsub.

Signed-off-by: B. Watson <urchlay@slackware.uk>

Signed-off-by: Willy Sudiarto Raharjo <willysr@slackbuilds.org>

gis/mapnik/mapnik.SlackBuild

@@ -77,7 +77,7 @@ tar xvf $CWD/$PRGNAM-$VERSION.tar.gz
 cd $PRGNAM-$VERSION
 
 # support build with current
-if [[ `icu-config --version` =~ ^61 ]] ; then
+if [[ $( icu-config --version ) =~ ^61 ]] ; then
   CUSTOM_DEFINES=-DU_USING_ICU_NAMESPACE=1
 fi