Initial work to build with WinHTTP on Windows.

* src/Makevars.win: Add -lwinhttp -lrpcrt4 to PKG_LIBS and
  -DGIT_WINHTTP to PKG_FLAGS. Needs testing if it builds.
Add configure.win to create import library for WinHTTP.dll
Update Makevars.win

* Add winhttp link directory and link ole32
Create import library for WinHTTP.dll on 64 bit build

* configure.win: Use dlltool to create import library for WinHTTP.dll
  on 64 bit build. Strip trailing @xx from src/winhttp/winhttp.def and
  save as src/winhttp/winhttp-x64.def.
PKG_LIBS in Makevars.win depends on 64 or 32 bit build
Improve clean rule in Makefile
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>
Merge branch 'build-with-winhttp-on-windows'

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