build: Adjust 'gen-AUTHORS' for 'distcheck' and when run from a tarball.

* Makefile.am (gen-AUTHORS): Don't do anything if '.git' is missing.  Remove
  "$(distdir)/AUTHORS" before invoking 'generate-authors.scm'.

Makefile.am

@@ -389,9 +389,12 @@ gen-ChangeLog:
 	fi
 
 gen-AUTHORS:
+	if test -d .git; then					\
+	  rm -f "$(distdir)/AUTHORS";				\
 	  $(top_builddir)/pre-inst-env "$(GUILE)"		\
 	    "$(top_srcdir)/build-aux/generate-authors.scm"	\
-	  "$(top_srcdir)" "$(distdir)/AUTHORS"
+	    "$(top_srcdir)" "$(distdir)/AUTHORS";		\
+	fi
 
 # Make sure we're not shipping a file that embeds a local /gnu/store file name.
 assert-no-store-file-names: