Fix race in 'make info/dir', and speed it up.

* Makefile.in (AWK, srcdir_doc_info_dir_inputs, info_dir_inputs):
New macros.
(clean): Remove info-dir.*.
(info_dir_deps): Depend on make-info-dir too.
Fix bug with wildcards that weren't expanded.
(${srcdir}/info/dir): Make sure info directory exists.
Don't call pwd; just redirect make-info-dir's stdout to temp file.
* build-aux/make-info-dir: Send output to stdout.
Take input file names from args.
Fix a "FIXME inefficient looping" by using awk's associative arrays.
Basically, this rewrites the script from scratch, and speeds up
'make info/dir' from 2.6s to 0.07s on my platform.