Avoid path problems at runtime when using cmake's ExternalProject_Add - c++

Question

I'm pretty new to cmake, but I am stuck with an odd behaviour that I'd like to understand and overcome.
I'm creating an executable that uses the Paho MQTT C library. The content of this executable is irrelevant, let's say it just connects to a broker for now, nothing fancy.
To compile my program, I use cmake with a CMakeLists.txt file, which does a few things:
use an ExternalProject block to pull the MQTT C lib from Github
declare the executables and the linked libs
Here it is :
project(myproject)
include(ExternalProject)
########## CMAKE VERSION AND FLAGS ##########
cmake_minimum_required(VERSION 3.1 FATAL_ERROR)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/bin)
########## MQTT EXTERNAL LIB ##########
set(MQTT_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR}/mqtt)
ExternalProject_Add(
project_mqtt
GIT_REPOSITORY https://github.com/eclipse/paho.mqtt.c
GIT_TAG v1.2.0
PREFIX ${MQTT_BUILD_DIR}
CMAKE_ARGS "-DCMAKE_INSTALL_PREFIX=${CMAKE_RUNTIME_OUTPUT_DIRECTORY}"
)
add_library(mqtt STATIC IMPORTED)
set(MQTT_LIB ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/lib/libpaho-mqtt3a.1.2.0.dylib)
set_property(TARGET mqtt PROPERTY IMPORTED_LOCATION ${MQTT_LIB})
add_dependencies(mqtt project_mqtt)
include_directories(${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/include)
link_directories(${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/lib)
########## EXECUTABLE ##########
add_executable(myproject ../src/main.cpp)
target_link_libraries(myproject mqtt)
Note that in the file I especially compile for Mac and I hardcoded the .dylib suffix but this is not a problem here
My folder structure is as follows :
myproject/
-- CMakeLists.txt
-- bin/
-- build/
-- src/
- main.cpp
My compilation workflow is very standard : cd build && cmake ../ && make.
My sources are in /src, I build everything in /build and my executable is written in /bin. The *.dylib are explicitely built locally in the project folder (in bin/lib) since I don't want to install them in the system (I want my project to be self-contained in a way).
It works well (I mean, it compiles).
But when I run the program, I get a dyld: Library not loaded: libpaho-mqtt3a.1.dylib error.
.. which is quite "normal", because my lib has been installed in bin/lib and my executable searches for them in its own folder (which is bin/). So for instance if I run : cd bin/lib && ../myproject it works perfectly well.
So, now here are my questions :
[Philosophically,] Is it a good way of doing this ? I want to use this external lib but I don't want to include the Paho lib sources in my code obviously, so I though ExternalProject was a good way to go. It compiles the lib at compile time before compiling my project and I find the process quite handy. I looked for a "package manager" for C++ but there does not seem to be a strong consensus (I'm looking at something like *composer* or *npm* for web apps), so I did not go this route.
Why, oh why, does my executable look for the library file in its own directory ./ ? Can I change that ? Is it some cmake configuration that I missed ? I guess that I could just use a cmake custom command to copy the library file into the /bin folder, but this looks like a hack to me. Isn't it ?
To be honest, I'd like to have no "path dependencies" in my executable, it's to say that the executable, when compiled on a specific system, could be put anywhere on this system and work flawlessly afterwards. I understand that means that the compiled Paho MQTT lib should be statically in my executable, but I wonder how to do that (or even what that really means, in fact). Is it possible ?
I may miss a whole part of how the C/C++ compile and external lib workflow works but I think I'm close to something quite robust, it's just this lib path thing at runtime that I would like to properly understand and fix.
I'm really open to suggestions on this
Thanks a lot !