Make the build of source tarball produce *.eln files

Make the build of source tarball produce *.eln files

* lisp/emacs-lisp/comp.el (batch-native-compile): Accept an
optional argument; if non-nil, place the .eln file as appropriate
for building a source tarball.

* doc/lispref/compile.texi (Native-Compilation Functions):
Document the new optional argument of 'batch-native-compile'.

* lisp/Makefile.in (.PHONY, $(THEFILE)n) [HAVE_NATIVE_COMP]: New
targets.

* src/Makefile.in (%.eln) [HAVE_NATIVE_COMP]: New recipe.
(all) [HAVE_NATIVE_COMP]: Add ../native-lisp to prerequisites.
(elnlisp) [HAVE_NATIVE_COMP]: New list of *.eln files.
(../native-lisp) [HAVE_NATIVE_COMP]: New recipe.

* src/verbose.mk.in (AM_V_ELN): New macro.