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