This ensures we use the same method in "make" as in "guix/self.scm".
* Makefile.am: Build guix/build/po.scm.
* build-aux/convert-xref.scm: New file.
* doc/local.mk (xref_command): Use it.
* guix/self.scm (translate-cross-references): Move it...
* guix/build/po.scm: Parse comments and flags separately to find fuzzy
flags.
(translate-cross-references): ...here.
(parse-tree->assoc): Ignore fuzzy entries.
