This happens at least in the following situation: Sage 9.1.beta9 on OS X, using the system's rather than Sage's Python, no pkg-config installed.

CC: @jhpalmieri @dimpase @orlitzky

Component: build: configure

Author: Matthias Koeppe

Branch: 0ac76c3

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29448