Added support for winpthreads library

Recent builds of MinGW64 runtime include the winpthreads library which
is not linked statically anymore and thus doesn't required non-portable
(de-)initialization function calls.
Install winpthreads DLL

Install winpthreads DLL for inclusion in Win32/Win64 installer. Requires
latest version of mingw-x-runtime packages (>=3.1.0).