ft2build.h is located here:
C:\\Program Files\\GnuWin32\\include
Initially, I made the same mistake as here:
Fatal
I spent hours trying to get matplotlib to install with the latest python 3, pip, setuptools everything... What finally worked for me was going to https://www.lfd.uci.edu/~gohlke/pythonlibs/#matplotlib downloading the wheel file and executing this in the same directory:
pip install matplotlib‑3.2.0rc2‑cp38‑cp38‑win_amd64.whl