Problem: Missing QUIET option causes a CMake warning

[kleisauke]

When libzmq is installed via a package manager, it causes a CMake warning when building cppzmq, which can be safely ignored.

There's no behavior change with this patch, because when libzmq isn't installed it will still raise a warning (find_package(ZeroMQ REQUIRED) is called a few lines further).

[coveralls]

Pull Request Test Coverage Report for Build 66

• 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 65:	0.0%
Covered Lines:	36
Relevant Lines:	64

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 66

• 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 65:	0.0%
Covered Lines:	36
Relevant Lines:	64

---

💛 - Coveralls