Platform-specific code.

The idea is that there are a few, platform-specific subdirectories. (I
expect one for Windows, one for generic Unix, and later (maybe) for
things like KDE/Gnome). Each of them contains at least a 'platform.h'.
At configure time, this file is made accessible as 'arch/platform.h'.

This directory also used to contain a 'rules.mak' to define extra
build rules. These are now defined within the P9/Settings-XXX file.

'platform.h' contains a 'struct Platform' with several static methods. 
In addition, 'platform.h' should contain a handful of #defines for
making certain things testable from the outside (details to follow).

NOTE: Windows platform support is mostly in 'win32-bcc55', because
it originally used Borland's Unix compatibility functions ('stat'
etc.). Now it uses native Win32 calls. New generic Windows code goes
in 'win32', the 'win32-xxx' directories just contain the files
generated by Autoconf under Unix.
