Minor improvements, mostly cmake

[emmenlau]

This PR makes it possible to build against a local system Google test and Google benchmark. It also adds a CMake install target, so that downstream users can use the library more easily with CMake. Using with CMake should work as:

find_package(uuid_v4)
target_link_libraries(MyFancyProject uuid_v4::uuid_v4)

[emmenlau]

This should be a quick thing to review, could it be considered?

[crashoz]

Sorry I'm very late on this! Thank you for the PR the install target is a nice addition.