Fix setup.sh bug on systems with no realpath program

setup.sh

 if [ -n "$BASH_VERSION" ]; then
+	command -v xargs > /dev/null || { echo >&2 "ERROR: xargs program not available."; }
+
 	if [ "`uname`" = "Darwin" ]; then
 		export TAPASCO_HOME=`dirname ${BASH_SOURCE[0]} | xargs cd | pwd`
 	else
-		export TAPASCO_HOME=`dirname ${BASH_SOURCE[0]} | xargs realpath`
+		command -v realpath > /dev/null || command -v readlink > /dev/null ||  { echo "ERROR: neither realpath nor readlink programs are available."; }
+		command -v xargs > /dev/null && command -v realpath > /dev/null && export TAPASCO_HOME=`dirname ${BASH_SOURCE[0]} | xargs realpath`
+		command -v xargs > /dev/null && command -v readlink > /dev/null && export TAPASCO_HOME=`dirname ${BASH_SOURCE[0]} | xargs readlink -f`
 	fi
 elif [ -n "$ZSH_VERSION" ]; then
-	export TAPASCO_HOME=`dirname ${(%):-%x} | xargs realpath`
+	command -v realpath > /dev/null || command -v readlink > /dev/null ||  { echo "ERROR: neither realpath nor readlink programs are available."; }
+	command -v xargs > /dev/null && command -v realpath > /dev/null && export TAPASCO_HOME=`dirname ${(%):-%x} | xargs realpath`
+	command -v xargs > /dev/null && command -v readlink > /dev/null && export TAPASCO_HOME=`dirname ${(%):-%x} | xargs readlink -f`
 else
-	echo "WARNING: unknown shell; need source setup.sh from the TaPaSCo root dir!"
+	echo "WARNING: unknown shell; need to source setup.sh from the TaPaSCo root dir!"
 	export TAPASCO_HOME=$PWD
 fi
 echo "TAPASCO_HOME=$TAPASCO_HOME"