CMake warning in regards to our use of CPACK_SOURCE_TGZ.

CMake Warning (dev) at /usr/local/Cellar/cmake/3.13.0/share/cmake/Modules/CPack.cmake:589 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.

For compatibility with older versions of CMake, option is clearing the
normal variable 'CPACK_SOURCE_TGZ'.
Call Stack (most recent call first):
CMakeLists.txt:122 (include)
This warning is for project developers. Use -Wno-dev to suppress it.