Find one of multiple packages

[osrf-migration]

Original report (archived issue) by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

---

Proposal

# find ignition-math3, or ignition-math4
# copy all variables prefixed with the found project name into variables prefixed with ${OUTPUT_VAR}
ign_find_package(
    OUTPUT_VAR
        some-math-version
    ONE_OF
        ignition-math4
        ignition-math3
    REQUIRED)

 # ...

target_link_libraries(my_lib ${some-math-version_LIBRARIES})

Developing on the default branch of ignition projects has one minor annoyance that occurs fairly frequently. The major version being bumped on one branch breaks the build of every downstream project. If the upstream project doesn't have a release then downstream must choose between everything-from-source builds being broken, or CI being broken. To get around this some projects have been finding either ignition-math3 or ignition-math4.