Installing ns3: What worked

Prerequisites

• XCode CLT (which you should have)
• QT 5. Install via brew install qt5. NOTE: if you already have an updated version of Qt, you'll need to pass a different qmake, and brew won't link this to your system. Brew will inform you of this, and you'll manually need to pass the path to qmake while building. More on this later.
• Python
• ? PyBindGen - I installed this via pip install pybindgen, but waf didn't pick it up. If anyone else succeeds, let me know in the comments.

Installation

• Download the source code, unzip with tar -xvf and cd into the directory
• Use the following command to build: ./build.py [--qmake-path=/usr/local/opt/qt@5/bin/qmake] --enable-examples --enable-tests -- --build-profile=optimized --disable-werror (Without the stuff in the square brackets, if you don't have a newer version of Qt preinstalled, and with the stuff but excluding the square brackets if you do have a newer version).

Aaaand you're good! To test, change into the ns-3.29 directory and do ./waf --run hello-simulator. You should see Hello Simulator printed. To test NetAnim, go to the netanim-3.108 folder and do ./NetAnim, and you should see the NetAnim GUI pop up.

Update: The python bindings are generated when you run waf using python2 (no idea why not with python3). To generate them, go to ns-3.29 and run python2 waf configure --build-profile=optimized --enable-examples --enable-tests --disable-werror and then ./waf build.