Add field 'Biarch: true' to DESCRIPTION

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update clean target in Makefile

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update configure.win

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add definition file for winhttp.dll

* src/winhttp (winhttp.def, winhttp-x64.def): Definition files for
  winhttp.dll to generate the import library in configure.win when
  building on Windows.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>