Build info files in builddir instead of srcdir (for PR automake/168),

Build info files in builddir instead of srcdir (for PR automake/168),
and prepare support for info files in subdirecties.
* automake.in (output_texinfo_build_rules): New function.
(handle_texinfo_helper): Use output_texinfo_build_rules, and
prepare to support subdir texinfos.  Define am__TEXINFO_TEX_DIR.
* lib/am/texi-vers.am: Build version.texi and stamp-vti in the
current directory, not $(srcdir).  Use %STAMPVTI% to name the stamp
file.
* lib/am/texibuild.am: Rewrite to output files in the current
directory, not $(srcdir).  Also, support both generic and
non-generic output.  Pass `-o $@' to makeinfo, texinfo, and
texi2dvi -- this is a partial fix for PR automake/339.
* tests/texinfo3.test: Make sure the `.texi:' rule is output.
* tests/texinfo10.test: Use set -e, so the checks are not ignored.
* tests/texinfo16.test: New file.
* tests/Makefile.am (TESTS): Add texinfo16.test.