Fix message using empty version variable

[zguangyu]

project(cppzmq VERSION ${DETECTED_CPPZMQ_VERSION}) will create a variable called cppzmq_VERSION instead of CPPZMQ_VERSION. This is the case under cmake v3.9.4 .

[coveralls]

Pull Request Test Coverage Report for Build 71

• 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%

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

Totals
Change from base Build 70:	0.0%
Covered Lines:	36
Relevant Lines:	64

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💛 - Coveralls

[kurdybacha]

Looks good to me. Thanks @zguangyu for providing the PR.