Reworked the MinGW32 build-dependencies script, it has been renamed to build-depkgs...

Moved the 3rd party package URLs to a separate file External-mingw32.  This allows us to update to new versions without having to modify the script and makes it easier to figure out which packages and which versions are used.

Added a new script build-depkgs-msvc.cmd which does the same thing as build-depkgs-mingw32 except they are built using Microsoft Visual C and placed in the depkgs-msvc directory.  There is also an External-msvc file to describe the packages.

Updated README.mingw32 with the updated instructions.

Added a couple of missing files to the installer for PostgreSQL.