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.
FreeBSD compatibility.

configure.ac:
    Remove the -P argument from a call to m4 as it's not supported
    everywhere.  We don't rely on -P in any case.

README.FreeBSD:
    Include more up-to-date information about Mercury on FreeBSD.