cmake: Silence configuration warning when MSVC is selected as generator.

[dmitrykos]

Silences CMake warning when MSVC is selected as generator:

CMake Warning at cmake/modules/FindJACK.cmake:43 (find_package):
  By not providing "Findpthreads.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "pthreads",
  but CMake did not find one.

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

    pthreadsConfig.cmake
    pthreads-config.cmake

  Add the installation prefix of "pthreads" to CMAKE_PREFIX_PATH or set
  "pthreads_DIR" to a directory containing one of the above files.  If
  "pthreads" provides a separate development package or SDK, be sure it has
  been installed.
Call Stack (most recent call first):
  CMakeLists.txt:138 (find_package)

[dmitrykos]

This PR could be merged painlessly but need one reviewer to approve.