Prevent rust language from building when cargo is
missing.

config/ChangeLog:

	* acx.m4: Add a macro to check for rust
	components.

ChangeLog:

	* configure: Regenerate.
	* configure.ac: Emit an error message when cargo
	is missing.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>