Create a namespaced Catch2::Catch target that is 'linkable' through
`target_link_libraries()` and export it so it is findable through
`find_package()`.

`find_package()` will find versions with the same major number and with
minor number >= requested.

This makes catch a lot easier to use in CMake-based projects. Whether it
is found using `find_package` or included in the client project as a
subdirectory, the client can include the catch headers per-target with
`target_include_directories(target PRIVATE Catch2::Catch).

Example usage:

    cmake_minimum_required(VERSION 3.1)

    # include Catch2 as subdirectory or installed package
    # add_subdirectory(Catch2)
    find_package(Catch2 VERSION 2.1.0 REQUIRED)

    add_executable(tests tests/catch_main.cpp)
    target_link_libraries(tests PRIVATE Catch2::Catch)

This means that only one place needs to work with warnings from
the deprecation of `std::uncaught_exception()` in C++17.

Closes #1162.