Problem: googletest is also installed into install destination

[kurdybacha]

Building and installing cppzmq brings googletest build artifacts to the
installation destination as well. For example someone building cppzmq
with:

cmake -DCMAKE_INSTALL_PREFIX=mydest .
cmake --build --target install

gets googletest headers and libraries in mydest.

Solution: remove googletest from install target

[coveralls]

Pull Request Test Coverage Report for Build 64

• 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 63:	0.0%
Covered Lines:	36
Relevant Lines:	64

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💛 - Coveralls