Export json11Config.cmake

[fegorsch]

Let CMake create and install json11Config.cmake allowing other CMake projects to easily import json11 as a CMake-target by setting json11_DIR to the corresponding location.

[artwyman]

It looks to me like this PR contains more changes than those described in the title and description. For instance the CMAKE_DEBUG_POSTFIX seems like a library-naming change which would break compatibility, so would need some justification.

Since we don't use CMake with json11 at Dropbox, I'd be curious to hear commentary from the other community-members who have contributed to this config, such as @4brunu and @admsyn.

[4brunu]

This PR goes behind my CMake knowledge, but I tested this change in one of my projects and it works.

[fegorsch]

I removed appending CMAKE_DEBUG_POSTFIX and will eventually open another PR for this.

[fegorsch]

I'm still using json11 and I'm still using CMake. A merge of this PR or a comment on how to make it ready-to-be-merged would really help me in my day-to-day worklife :)