Detect when msvc 6.0 is in use without PSDK installed, intentionally

Detect when msvc 6.0 is in use without PSDK installed, intentionally
fail to build when this happens, and show an appropriate error.

The brave of heart can circumvect this. Defining ALLOW_MSVC6_WITHOUT_PSDK
in lib/config-win32.h, although absolutely discouraged and unsupported,
this will allow the die hard MSVC hacker to build in such a discouraged
environment.

The actually supported 'fix' is to install 'February 2003 Platform SDK'
a.k.a. 'Windows Server 2003 PSDK' which can be freely downloaded from
http://www.microsoft.com/msdownload/platformsdk/sdkupdate/psdk-full.htm