eliminate CMP0077 cmake warning building macOS package
eliminate CMP0077 cmake warning building macOS package

Gets rid of warning when building RStudio package on macOS:
```
CMake Warning (dev) at /usr/local/Cellar/cmake/3.14.0/share/cmake/Modules/CPack.cmake:548 (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_BINARY_DRAGNDROP'.
  Call Stack (most recent call first):
    package/osx/CMakeLists.txt:46 (include)
```

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