Fix multi-file installation for files in srcdir.

Fix multi-file installation for files in srcdir.

For files that can live in the build tree or the source tree,
we have to expect that prepending `$(srcdir)/' to each name
will let the install command line exceed length limitations.
Thus use a new macro, $(am__base_list), to install in chunks
of 40 files, similar to what we do for nobase_* files.  There,
lower the limit to 40, too.

* lib/am/inst-vars.am (am__install_max): Lower to 40.
(am__base_list): New macro.
* lib/am/data.am (install-%DIR%%PRIMARY%) [%?BASE%]: Use it.
* lib/am/texinfos.am (install-dvi-am, install-html-am)
(install-info-am, install-pdf-am, install-ps-am): Likewise.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>