Improve generated scripts cgal_test_with_cmake:

- Do not try to compile targets that have not been configured. Hopefully
    cmake gives us "make help".
  - Do not try to clean if nothing has been built.

Scripts/developer_scripts/create_cgal_test_with_cmake

@@ -61,6 +61,7 @@ cat << EOF
   if [ -z "\${MAKE_CMD}" ]; then
     MAKE_CMD=make
   fi
+  NEED_CLEAN=
 EOF
   echo
   header "configure"
@@ -144,7 +145,12 @@ EOF
     if [ -n "`grep '\<main\>' $file`" ] ; then
       tmp=`basename $file .C`
       tmp=`basename $tmp .cpp`
-      echo "  compile_and_run $tmp"
+      cat <<EOF
+if  \${MAKE_CMD} -f Makefile help | grep -q $tmp; then 
+  compile_and_run $tmp
+  NEED_CLEAN=y
+fi
+EOF
     fi
   done
 cat << EOF
@@ -154,8 +160,10 @@ fi
 # The clean target generated by CMake under cygwin 
 # always fails for some reason
 #
-if ! ( uname | grep -q "CYGWIN" ) ; then
-  make -fMakefile clean
+if [ -n "\${NEED_CLEAN}" ]; then 
+  if ! ( uname | grep -q "CYGWIN" ) ; then
+    \${MAKE_CMD} -fMakefile clean
+  fi
 fi
 EOF