Allow library output name to be changed externally

[eldruin]

Internally we rename the libraries we integrate as to avoid interference with the library versions already available on the target systems by giving them a prefix.
This PR allows users to specify, for example:

-DZMQ_OUTPUT_BASENAME:STRING=MYORG_zmq

I used BASENAME as the flavor and/or version are appended later.

Alternatively, it would be also fine for us to add a prefix variable like OUTPUT_NAME_PREFIX or so but I was unable to find a good name that cannot be confused the target PREFIX property (e.g. lib)

[bluca]

Thanks for the PR - please add a commit with a relicense statement https://github.com/zeromq/libzmq/blob/master/RELICENSE/README.md

[eldruin]

@bluca done