pyproj: Linux pip install gives "Proj executable not found. Please set PROJ_DIR variable."

Installation method/steps

pip install pyproj

Environment Information

• pyproj version you are attempting to install: 2.4.1 (this is not an issue for 2.4.0)
• Tested with 3.5 and 3.6
• Tested with Linux Mint 18 and Ubuntu Xenial

Error

 Running setup.py (path:/tmp/pip-install-qfelb_w6/pyproj/setup.py) egg_info for package pyproj
    Running command python setup.py egg_info
    Proj executable not found. Please set PROJ_DIR variable.

Since 2.4.1 release pip install won’t work unless you have a more recent version of pip. pip 18.1 for example will fail because it refuses to download the wheel packages and then will try to compile the sources.

Skipping link https://files.pythonhosted.org/packages/a5/b3/a3e00996681ef6e2e19cf367ff57e5e83ad8357f398b0cb71dab5cc8b756/pyproj-2.4.1-cp35-cp35m-manylinux2010_x86_64.whl#sha256=a1422da59673ca5bf56d89cf8e2a3cb2c9868bea27bbee6600ae603d322316a8 
 (from https://pypi.org/simple/pyproj/) (requires-python:>=3.5); it is not compatible with this Python

Fix

The solution is to upgrade pip to a later version (pip install --upgrade pip)

You can close this issue if you think this is not a bug worth fixing. I created it mostly so users with the same issue may hopefully find this.