Linking ns3 as a third party dependency

[Jared Barnak]

Typically, when consuming third party packages, you only need to: 1.) Find the package, 2.) link the library, and 3.) include the public headers

```
cmake_minimum_required(VERSION 3.26)
project(net CXX)

add_executable(net main.cpp)

find_package(ns3 REQUIRED PATHS /home/jared/Installs/ns3/cmake-cache)

target_link_libraries(net PRIVATE
    ns3
)

target_include_directories(net PRIVATE /home/jared/Installs/ns3/build/include)
```

`find_package` looks for `ns3Config.cmake`, and as it is not part of the path, I
hard coded the value.

And it'll find  `ns3Config` just fine, but it won't be able to find `ns3Targets.cmake` which is defined within the `ns3Config.cmake` i

It has it located at:

```
/home/jared/Projects/ns-3-dev/cmake-cache/ns3Targets.cmake
```

when it's actually located at:

```
build/CMakeFiles/Export/2a81891a6eb0eac8fbb86ed7caf9a458/ns3Targets.cmake`
```

I would ideally like to create a minimal project and use the following headers:


```
#include "ns3/applications-module.h"
#include <ns3/core-module.h>
#include <ns3/flow-monitor-module.h>
#include <ns3/internet-module.h>
#include <ns3/point-to-point-module.h>
```

TIA!

[Gabriel Ferreira]

ns3Targets.cmake and ns3Config.cmake get created in the appropriate places when installed.
If you want to use it without installing it, just make it an ExternalProject and consume the libraries directly. 

[Jared Barnak]

It seems I'm still having trouble even when using `ExternalProject`?

```
cmake_minimum_required(VERSION 3.25.2)
project("NetSim")

add_executable(netsim src/main.cpp)

include(ExternalProject)
ExternalProject_Add(ns3
  GIT_REPOSITORY    g...@gitlab.com:nsnam/ns-3-dev.git
  #
  # BUILD_COMMAND     ${CMAKE_SOURCE_DIR}/build/ns3-prefix/src/ns3/ns3 configure && cmake --build ${CMAKE_SOURCE_DIR}/build/ns3-prefix/src/ns3/ns3
)

set(LIB_PATH ${CMAKE_SOURCE_DIR}/build/ns3-prefix/src/ns3/build/lib)
find_library(NS3_CORE libns3-dev-core-default HINTS ${LIB_PATH})

target_link_libraries(netsim PRIVATE
   ${NS3_CORE}
)
```

[Gabriel Ferreira]

No need to use find_library, it's already available with libcore, libnetwork, etc

You can also add ns3:: prefix. 

[Jared Barnak]

Mind providing a minimal example?

I've tried both:

```
cmake_minimum_required(VERSION 3.25.2)
project("NetSim")

add_executable(netsim main.cpp)

include(ExternalProject)
ExternalProject_Add(ns3
  GIT_REPOSITORY    g...@gitlab.com:nsnam/ns-3-dev.git

)

target_link_libraries(netsim PRIVATE
  ns3::libcore
  ns3::libnetwork
)
```

And

``
cmake_minimum_required(VERSION 3.25.2)
project("NetSim")

add_executable(netsim main.cpp)

include(ExternalProject)
ExternalProject_Add(ns3
  GIT_REPOSITORY    g...@gitlab.com:nsnam/ns-3-dev.git

)

target_link_libraries(netsim PRIVATE
  libcore
  libnetwork
)
```


And building with:
```
cmake -B build
cmake --build build
```

And I either can't find the target `ns3::libcore` or  I get a linking error

```
`/usr/bin/ld: cannot find -llibcore: No such file or directory
/usr/bin/ld: cannot find -llibnetwork: No such file or directory
collect2: error: ld returned 1 exit status

```

Additionally, I'm struggling to get `find_package` to work.