Change build system to support new and old GNU automake

Change build system to support new and old GNU automake

Starting with GNU automake 1.12, the "de-ANSI-fication support" has been
removed, which ngIRCd used to enable building itself on very old systems.

Now the problem is, that using automake >= 1.12 isn't working because of
the now unsupported M4 macros. Therefore the solution that this patch
implements is to dynamically generate the automake input files with our
own ./autogen.sh script:

  configure.ng => configure.in
  Makefile.ng => Makefile.am

This is quite an ugly approach, but it works and enables us to:

  1. use current automake >= 1.12 for development and "private builds",
  2. still build distribution archives using automake 1.11.x that have
     "de-ANSI-fication support" enabled in the generated Makefile's.

And if you are using Makefile's generated with a automake version newer
than 1.11.x (without "de-ANSI-fication support"), the ./configure script
warns you not to use this generated build system to generate distribution
archives.

Drawback of this patch: you MUST use our autogen.sh script, you can't call
the autoconf/automake commands directly any more; but autoreconf should
still work ...