Disable install targets in CMakeLists.txt

[huberdavid]

We use azmq as a submodule in our projetct. AZMQ header files are added to the cmake install targets in the following line:

# install the public headers
install(DIRECTORY azmq DESTINATION include)

When using the cpack packager, it encapsulates these header files with our package unintentionally. We need to have an option that can disable install targets for azmq.