Avoid overwriting cmake output directories variables

**Background**
Indigo works well as a cmake subproject using add_subdirectory(). However it overwrites the global cmake variables:
CMAKE_ARCHIVE_OUTPUT_DIRECTORY
CMAKE_LIBRARY_OUTPUT_DIRECTORY
CMAKE_RUNTIME_OUTPUT_DIRECTORY
And therefore the binaries end up outside of the parent project's runtime directory.

**Solution**
Indigo should check if these variables are defined before setting them. If they are already defined, then don't change them.

**Alternatives**
Avoid using these cmake global variables and use RUNTIME_OUTPUT_DIRECTORY.

**Additional context**
This is a very low priority change, but it would make indigo more portable / easier to incorporate into other projects.