Support full range of sqlite build/find options on Windows

Trunk (r35537) doesn't currently support building/finding sqlite on Windows the
same way it does on *nix.  On the latter you have four options:

  1) Place the sqlite amalgamation in ./sqlite-amalgamation where
     it will automatically be found and built.
  2) Place the sqlite amalgamation in some other directory, point
     to that directory with the --with-sqlite config option, and
     Subversion will build it.
  3) Specify a sqlite installation to use with the --with-sqlite
     config option.
  4) Let Subversion find an installed SQLite.

Windows on the other hand supports only one option at the moment:

  Use the --with-sqlite option to gen-make.py to point to a
  directory with this structure and contents:
     
    inc/
    inc/sqlite3.h
    lib/
    lib/sqlite3.lib
    
These must be built manually, prior to running gen-make.py, the build system
will not build them on Windows.

We should support at least options 1-3 on Windows.