Fix configure's .NET library version test.

m4/mercury.m4:
	Check for a CLI interpreter *before* working out what
	C# compiler to use.

	If we have a CLI interpreter, then use that to run the
	test program we build for determining the .NET library version.
	(Previously, we just tried to execute the .exe directly and this
	didn't work on non-Windows systems.)

	Delete an unused variable.