surpport for build in msys2

[shishirong]

msys2 don't have Pthreads4W but it have pthread/libwinpthread, how to compatible

CMake Error at CMakeLists.txt:75 (find_package):
  By not providing "FindPthreads4W.cmake" in CMAKE_MODULE_PATH this project
  has asked CMake to find a package configuration file provided by
  "Pthreads4W", but CMake did not find one.

  Could not find a package configuration file provided by "Pthreads4W" with
  any of the following names:

    Pthreads4WConfig.cmake
    pthreads4w-config.cmake