minor additions to build system (info files in share/)

Hello,

I've created Makefile.am in some share/ directories to cause
info files to be built and modified configure.in to generate the
corresponding Makefiles. With these changes, ./configure,
make, make install, and make dist-gzip all succeed on my
development box (Linux).

Should be OK on other systems but since modifying the
build system in any way has the potential to cause trouble
I'm mentioning it here.

I have already committed some changes to cl-info.lisp and
init-cl.lisp so that share directories are searched for info.
Now "? read" for example finds topics in the share directories
which contain info files (namely share/contrib/numericalio and
share/contrib/stringproc). I'm planning to expand the coverage
of info in share/.

Hope this helps,
Robert Dodier