optional and test dependencies/targets

[ryanmrichard]

Is your feature request related to a problem? Please describe.
Having dependencies/targets which are only built sometimes is pretty common. Examples are dependencies/targets needed only for testing and dependencies/targets for optional features. Handling these "sometimes" objects introduces additional logic into the CMakeLists.txt which could be avoided by tagging the object as optional/testing. It's worth noting that the distinction between optional and testing boils down to whether they are installed or not (testing is usually not installed).

Describe the solution you'd like
A brute force approach would be to do something like:

cmaize_find_or_build_xxx_yyy(
)

where xxx would be optional or testing and yyy would be dependency, library, or executable.

Describe alternatives you've considered
Using control flow logic in my CMakeLists.txt files.

From #121:

• Add cmaize_find_or_build_test_dependency
• Add cmaize_add_test_library
• Add cmaize_add_test_executable
• Add cmaize_add_test (have cmaize_add_tests redirect to it)