Improve build system and autodetection.

Improve build system and autodetection.
* Make ./configure standalone, that's what people expect.
* Let people write a ./configure.local from scratch, not edit existing files.
* Autodetect wchar, sqlite3, and manpath and act accordingly.
* Autodetect the need for -L/usr/local/lib and -lutil.
* Get rid of config.h.p{re,ost}, let ./configure only write what's needed.
* Let ./configure write a Makefile.local snippet, that's quite flexible.