On unix-alikes, this function is run in .onLoad() to check that version 6 functionality is
available. On Windows, the load process sets the data file location with the version 6 API, and that
is used as a test instead.
If 'PROJ' library version 6 is not available, the package still compiles and installs
but is not functional.
The lack of function can be simulated by setting
options(reproj.mock.noproj6 = TRUE), designed for use with the reproj package.