Improvement: Declare unused variables in configure/test_cppzmq_features.cpp as such

When compiling cppTango with -pedantic -Wall -Wunused -Werror on macOS, then the try_compile for cppzmq already fails due to unused variables in the test code.

This can be avoided by declaring the variables as unused by adding __attribute__((unused)). The test code's behaviour does not change at all, only is the compiler instructed to not warn about the unused variables in the test code.