Support for re-running autoconf in the absence of the autoconf macro

Support for re-running autoconf in the absence of the autoconf macro
files which provide auto-detection of GTK 1 and GTK 2. This makes it
easier for casual PuTTY developers to rerun autoconf for their own
purposes without having to install obscure extra packages. Obviously
the resulting configure script will not know how to detect whichever
version of GTK they didn't have support for, so it won't be product-
quality by my standards, but it should be good enough that they can
prepare unrelated patches to send to us.

[originally from svn r9227]