use pkg-config to get it.

configure tarball is here:
http://users.ox.ac.uk/~coml0531/sage/configure-2bd1bda5f85b8f8e4a7fc7499f8529795099160a.tar.gz

CC: @orlitzky @jdemeyer @kiwifb

Component: build: configure

Author: Dima Pasechnik

Branch/Commit: 8c97d9b

Reviewer: François Bissey, Erik Bray

Issue created by migration from https://trac.sagemath.org/ticket/27825