Problem: tests are always building

[kleisauke]

Add the ability to explicitly not build tests. Option defaults to ON to keep the current behavior, but give the user the option to disable building of tests.

This option is similair to ZMQ_BUILD_TESTS, but just for cppzmq.

[coveralls]

Pull Request Test Coverage Report for Build 40

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 56.25%

---

Files with Coverage Reduction	New Missed Lines	%
zmq.hpp	2	64.38%

Totals
Change from base Build 37:	0.0%
Covered Lines:	36
Relevant Lines:	64

---

💛 - Coveralls

[kurdybacha]

looks good too me, thanks!