Fix for Issue 90

This allows NAnt to build on Mono 3.0+

Successfully tested on the following setups:

* Windows 7
  * Net 2.0
  * Net 4.0
* OpenBSD 5.2
  * Mono 4.0
* OpenSuSE 12.1
  * Mono 2.0

The only thing I would like to try is Mono on Windows.  I haven't been able to get it working on my Win box because I had trouble getting `make` working with the Mono installation in Windows through cygwin.