Provide sane diagnostic when Boost.Jam cannot be built

[SVN r54194]

bootstrap.sh

@@ -215,7 +215,13 @@ rm -f config.log
 if test "x$BJAM" = x; then
   echo -n "Building Boost.Jam with toolset $TOOLSET... "
   pwd=`pwd`
-  cd "$my_dir/tools/jam/src" && ./build.sh "$TOOLSET" > config.log 2>&1
+  (cd "$my_dir/tools/jam/src" && ./build.sh "$TOOLSET") > bootstrap.log 2>&1
+  if [ $? -ne 0 ]; then
+      echo
+      echo "Failed to build Boost.Jam" 
+      echo "Consult 'bootstrap.log' for more details"
+      exit 1
+  fi
   cd "$pwd"
   arch=`cd $my_dir/tools/jam/src && ./bootstrap/jam0 -d0 -f build.jam --toolset=$TOOLSET --toolset-root= --show-locate-target && cd ..`
   BJAM="$my_dir/tools/jam/src/$arch/bjam"