Use find to be compatible with open_basedir

When using open_basedir, commands such as `which`or `command`can fail because they might try to look outside of the restricted perimeter.
When using `find`, we can give the command a couple of standard directories to look in.

The better solution would be to adopt a mechanism similar to what has been done for office documents where a configuration parameter is used to indicate the path where the executable is to be found.