added MSVC6 caveats.

   See the separate INSTALL.devcpp file for details.

   MSVC 6 caveats

   --------------

   If you use MSVC 6 it is required that you use the MS PSDK:

   http://www.microsoft.com/msdownload/platformsdk/sdkupdate/psdk-full.htm

   The headers and libs provided with MSVC 6 are now more than 10 years old

   and not sufficient for a build of libcurl / curl with common features!

   Without the PSDK you will run into these issues:

   - LDAPv3 is not supported --> breaks compilation

   - IPv6 support is incomplete --> breaks compilation

   - socklen_t type is missing --> breaks compilation

   If you really need to compile without PSDK then you can workaround these

   issues by disabling LDAP and IPv6 support, and adding a line:

   typedef int socklen_t;

   to the winsock2.h header. This is however not recommended, and you should

   use the PSDK whenever possible!  

   MSVC from command line

   ----------------------