Typo in killall5.c comment
Add --enable-* options to configure.ac for old src/Makefile settings.

Previously the src/Makefile file contained blocks per distribution to
group together the necessary CFLAGS and other settings.  Replaced those
with --enable/--disable options to the ./configure script.
Start an INSTALL file.

This is not the typical autoconf INSTALL file either.  It's intended to
be useful.
Get version and build date strings consistent.

Define these at build time via autoconf, include them from config.h.
Remove static version strings from the different programs.
Remove __linux__ #ifdef blocks.

We don't need these because we're always on Linux.
Remove __FreeBSD__ and other BSD-specific OS #ifdef blocks.