gtest directory missing in tarball

[ryandesign]

The source code tarball does not contain the gtest directory but CMakeLists.txt tries to use the nonexistent gtest directory if it detects that the gtest program is installed, leading to this cmake failure:

CMake Error at CMakeLists.txt:315 (add_subdirectory):
  add_subdirectory given source "gtest" which is not an existing directory.

-- Configuring incomplete, errors occurred!

This was reported to MacPorts here: https://trac.macports.org/ticket/67333

Please include the gtest directory in the tarball in future versions of pupnp.

[ryandesign]

I see this isn't the first time files have been missing from the tarball (#280, #419).

[mrjimenez]

Please check if it works now.