Disable install targets in CMakeLists.txt

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.