add support for `make install`
add support for `make install`

use
```
cmake -DCMAKE_INSTALL_PREFIX=<path> .
make install
```

alexeymarkov/RxCpp,ReactiveX/RxCpp,ReactiveX/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp