build: Do not require "build-aux/guix.scm".

When building Cuirass with 'guix build -f build-aux/guix.scm' we don't want to
add "build-aux/guix.scm" in the repository snapshot since it is considered as
metadata.  As a consequence we can't make 'autoconf' require it.

* configure.ac (AC_REQUIRE_AUX_FILE): Remove "build-aux/guix.scm".
* Makefile.am (EXTRA_DIST): Only distribute it.