Use a version check to provide backwards comatible CMake imported target names

[chuckatkins]

Fixes #1243

[coveralls]

Coverage remained the same at 100.0% when pulling 3b1a5ca on chuckatkins:fix-target-namespace-backward-compatibility into 99939d6 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot for the quick reaction!

[SylvainCorlay]

I like the fix. Did not know about PACKAGE_FIND_VERSION.

[SylvainCorlay]

@nlohmann @chuckatkins this does not appear to work. People are complaining that they cannot build against master and getting.

CMake Error at nlohmann_jsonConfig.cmake:30 (add_library):
  add_library cannot create ALIAS target "nlohmann_json" because target
  "nlohmann_json::nlohmann_json" is IMPORTED.
Call Stack (most recent call first):
  CMakeLists.txt:33 (find_package)

I think that we should revert that change.

[chuckatkins]

Bad on my part :-(. I'll push a fix today, but also add a test to validate import configs to prevent future breakage.

[chuckatkins]

@ericu65 this should be fixed by #1260

[ericu65]

@chuckatkins Thanks. On first look this would seems to fix it. Looking forward to a merge.