Portability enhancements to make better use of autoconf. All system

#includes (apart from the Linux kernel syscall ickery in du.c) have
been centralised into agedu.h and been made conditional on the
relevant header files being shown to exist by configure. My hacky
personal config.h has been removed and the static development
Makefile now runs the real configure script when necessary. There
are a couple of other tweaks which work around buggy printfs on
Solaris.

git-svn-id: svn://svn.tartarus.org/sgt/agedu@8281 cda61777-01e9-0310-a592-d414129be87e