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.

Makefile.am

@@ -103,6 +103,7 @@ EXTRA_DIST = \
   bin/cuirass.in \
   bin/evaluate.in \
   bootstrap \
+  build-aux/guix.scm \
   src/cuirass/config.scm.in \
   $(TESTS)
 

configure.ac

@@ -24,7 +24,6 @@ AC_INIT([Cuirass],
 AC_CONFIG_SRCDIR([bin/cuirass.in])
 AC_CONFIG_AUX_DIR([build-aux])
 AC_REQUIRE_AUX_FILE([git-version-gen])
-AC_REQUIRE_AUX_FILE([guix.scm])
 AC_REQUIRE_AUX_FILE([test-driver.scm])
 AM_INIT_AUTOMAKE([foreign subdir-objects -Wall])
 AM_SILENT_RULES([yes])		# enables silent rules by default