Remove support for bcc and msvc standalone libpq builds

Remove support for bcc and msvc standalone libpq builds

This removes the support for building just libpq using Borland C++ or
Visual C++. This has not worked properly for years, and given the number
of complaints it's clearly not worth the maintenance burden.

Building libpq using the standard MSVC build system is of course still
supported, along with mingw.