Provide benchmark:: Aliased CMake Library

[jwillikers]

It's common to provide an aliased version of libraries that include the import prefix to make switching between find_package and add_subdirectory calls effortless. Because the benchmark::benchmark and benchmark::benchmark_main targets do not have aliased targets, users must switch between benchmark::benchmark and benchmark in target_link_libraries.
Adding an aliased library with CMake is as easy as:

add_library(benchmark::benchmark ALIAS benchmark)

[keith-bennett-gbg]

Note this was also relevant to me when building something depending on benchmark using find_package with a system installed benchmark (cmake && sudo make install) and then migrating to a user-installed (cmake -DCMAKE_INSTALL_PREFIX=${HOME} && make install). Turns out the project depending on benchmark was linking to benchmark and was finding the system installed benchmark but would break when building the user installed benchmark (fatal error: benchmark/benchmark.h: No such file or directory). Using benchmark::benchmark fixed the user installed benchmark and the system installed package consistently worked too.

That makes sense and the bug was obviously on my end. Remember, cmake links against benchmark alone if it can't find the target, and the compiler found the system-installed library. CMake did not add any include directories because the correct target was not used, but because benchmark was system installed, the compiler found the system-installed headers too. Migrating to a user install meant that the compiler couldn't pick up the slack and showed that CMake wasn't telling me that I'm not correctly using the benchmark target. I really wish CMake could tell and warn or error when that happens.

I think it would be nice if there's a few different examples and includes benchmark::benchmark with find_package(benchmark REQUIRED) instead of leaving me to (incorrectly) guess as it is currently. I'd considered using add_subdirectory instead to work around the problem but found this issue while searching for any known issues with installing benchmark. It gave me the insight to try using target_link_libraries(mytarget benchmark::benchmark) which solved the problem: cmake now recognized the target and automatically added the necessary include directory to mytarget.

Specifically: the README includes Installation with some example of installing benchmark with cmake. It also includes Usage showing how to use Benchmark in C++ code. I think an improvement could be made here to demonstrate using benchmark in cmake, showing an example of find_package and add_subdirectory.

[LebedevRI]

SGTM, feel free to submit a pull request.

[jwillikers]

@LebedevRI @keith-bennett-gbg I am working on a pull request now 👍