Examples: Adapt to system-packaged DDS

Distribution policies forbid installation of environment scripts at the
install prefix. So, e.g. in the Fedora DDS package the DDS_env.sh is
installed to /usr/bin and available via $PATH.

examples/dds/fairmq-start-ex-dds.sh.in

@@ -18,7 +18,12 @@ cleanup() {
   echo "CLEANUP PERFORMED"
 }
 
-source @DDS_INSTALL_PREFIX@/DDS_env.sh
+if [[ -e "@DDS_INSTALL_PREFIX@/DDS_env.sh" ]]; then
+  source @DDS_INSTALL_PREFIX@/DDS_env.sh
+elif command -v DDS_env.sh &> /dev/null; then
+  source DDS_env.sh
+fi
+
 export PATH=@BIN_DIR@:$PATH
 
 plugin=${1:-localhost}