add the ability to set the executable used in .net tasks. Why do this
(a) so you can refer to versions off the path (useful for multiple versions)
(b) so you can use other implementations than the windows one.
Specifically, Mono Csharp compiler, msc, apparently uses the same syntax as csc if desired. We will have to test this... I suppose we could test Rotor too, but I cant be bothered
git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@275000 13f79535-47bb-0310-9956-ffa450edef68