Remove dependency on GNU Make and reliance on /usr/bin/env.

Remove dependency on GNU Make and reliance on /usr/bin/env.

Both were introduced in commit c55cb58a ("integrate guile-lib docs").

* doc/ref/Makefile.am ($(snarf_doc)scmfiles): Rename target to...
  ($(snarf_doc)-scmfiles): ... this.  Change to generate a makefile
  snippet containing a variable definition.  Automake-include the
  generated snippet, `standard-library.am'.
  (EXTRA_DIST): Add `$(snarf_doc).am'.
  (CLEANFILES): Remove `$(snarf_doc)scmfiles'.

* doc/ref/make-texinfo.scm: Remove executable bit and "#!/usr/bin/env".

* doc/ref/standard-library.am: New file.