Avoid configure failures when the lib/mono/2.0 directory does not exist.

NET 2.0 support in mono is going away[1], and fsharp includes configure checks which break if lib/mono/2.0 directory does not exist. The attached patch fixes that.

[1]:
http://lists.ximian.com/pipermail/mono-devel-list/2014-October/042145.html