Tools: jsbsim script should not re-install jsbsim if it's already installed

It's called unconditionally from other script/s that may be run repeatedly

Tools/scripts/build-jsbsim.sh

@@ -2,9 +2,20 @@
 
 set -e
 set -x
+echo "---------- $0 start ----------"
 
-rm -rf jsbsim
-git clone https://github.com/tridge/jsbsim.git
-cd jsbsim
-./autogen.sh
-make -j2
+# don't waste time rebuilding jsbsim if we already have a working copy of it
+# this can save ~10 or minutes on some hardware
+JSBBINARY=~/jsbsim/src/JSBSim
+if [ -f $JSBBINARY ]; then
+    echo "$JSBBINARY exists, skipping build. (to force rebuild then remove this file and re-run) "
+else
+    echo "$JSBBINARY does not exist, building it in your home folder:"
+    cd ~ 
+    rm -rf jsbsim
+    git clone https://github.com/tridge/jsbsim.git
+    cd jsbsim
+    ./autogen.sh
+    make -j2
+fi
+echo "---------- $0 end ----------"