Better use of flex home

This was bugging me, so I fixed it the way I think it should work.    Set the environment variable on your system, e.g. 

```
export FLEX_HOME=/Applications/flex_sdk_4.6
```
in your /etc/.profile or ~/.profile or similar, and never have to hard code it in the build.xml file ever again.

I think this would work in windows too if you set the environment variable via Manage Computer, but I haven't tested it there.