download snap7 archive only, if not already available

snap7/build.sh

@@ -4,9 +4,13 @@ set -e
 SNAP7_VERSION=1.4.0
 SNAP7_FOLDER=snap7-full-${SNAP7_VERSION}
 
-rm -rf snap7-full*
-wget http://downloads.sourceforge.net/project/snap7/${SNAP7_VERSION}/${SNAP7_FOLDER}.tar.gz
-tar -vxzf ${SNAP7_FOLDER}.tar.gz
-mv ${SNAP7_FOLDER} snap7-full
+archiveFile=${SNAP7_FOLDER}.tar.gz
+if [ ! -f "$archiveFile" ]
+then
+   rm -rf snap7-full*
+   wget http://downloads.sourceforge.net/project/snap7/${SNAP7_VERSION}/${archiveFile}
+   tar -vxzf ${archiveFile}
+   mv ${SNAP7_FOLDER} snap7-full
+fi
 cd snap7-full/build/unix
 make -f x86_64_linux.mk