cmake ExternalProject_Add how to download URL non-tar file?

How does one set up an External project to download a link that isn't a .tgz file?

For example Catch provides a release that is a single header distribution. I would like to just download this rather than the git repo or the .tgz release. But I haven't figured out how to tell CMake to do this.

I would like to do something like:

Include(ExternalProject)
ExternalProject_Add(
    catch
    PREFIX ${CMAKE_BINARY_DIR}/catch
    URL https://github.com/philsquared/Catch/releases/download/v1.9.6/catch.hpp ${CMAKE_BINARY_DIR}/catch
    CONFIGURE_COMMAND ""
    BUILD_COMMAND ""
    INSTALL_COMMAND ""
    LOG_DOWNLOAD ON
)

This fails as URL assumes that url is a compressed tar file. I have tried various variants of the DOWNLOAD_COMMAND without any success either.