makefile: replace unnecessary use of xargs

Questions:

  - Is there any value in quoting filenames in this context? After reading [GNU Make meets file names with spaces in them][1] I'm don't think there's much hope of correctly handling filenames containing spaces.

  - Is there a reason to favour this two-step process over `$(COFFEE) -co $(@D) $<`? The latter would obviate the need for `mkdir` on our part.


[1]: http://www.cmcrossroads.com/article/gnu-make-meets-file-names-spaces-them