Create a namespaced CMake target for ApprovalTests

[claremacrae]

For details, see the really clear explanation by @athrun22 in: #69 (comment)

[claremacrae]

Here's the explanation, copied from the original issue:

@claremacrae, a namespaced CMake target for ApprovalTests would be good to create before this documentation is done. By adding an aliased CMake target for ApprovalTests that prefixes a namespace, users can switch between using an installed version of the library - if support for find_package is added - and add_subdirectory / FetchContent without having to change the ApprovalTests library name. That's dense when I put it into words 🙄

An example might make this more clear. 🤔
The following example links to a hypothetically installed version of ApprovalTests, though this not supported at the moment.

# Use an installed version of ApprovalTests
find_package(ApprovalTests 7.0.0 REQUIRED)
add_executable(my-test PRIVATE main.cpp)
# According to standard practice, imported targets are prefixed with a namespace
target_link_libraries(my-test PRIVATE ApprovalTests::ApprovalTests)

This links to our current setup of ApprovalTests when using add_subdirectory.

add_subdirectory(ApprovalTests)
add_executable(my-test PRIVATE main.cpp)
# Notice the target name is different. This requires the user to change the library name.
target_link_libraries(my-test PRIVATE ApprovalTests)

This links to our current setup of ApprovalTests when using add_subdirectory after adding an aliased target.

add_subdirectory(ApprovalTests)
add_executable(my-test PRIVATE main.cpp)
# Target name is the same and this requires no change by the user.
target_link_libraries(my-test PRIVATE ApprovalTests::ApprovalTests)

Aliasing the ApprovalTests library in CMake is as simple as adding the following somewhere after defining the ApprovalTests interface library, probably in the same directory where it is defined.

add_library(ApprovalTests::ApprovalTests ALIAS ApprovalTests)

[claremacrae]

I'd been wondering whether to add an aliased target - this is a good explanation of one reason to do so, that's new to me...

A reason that I was aware of is that in target_link_libraries(), CMake won't warn if ApprovalTests is not found, but it will warn if an library alias isn't found - so that's kinder to users...

[jwillikers]

@claremacrae Right you are! I use alias libraries internally for this very reason.

[claremacrae]

@jwillikers, does the above change look OK? Any feedback on the documentation change? Many thanks.