While PKG_PROG_PKG_CONFIG checks the version of pkg-config available at
build time, it doesn't check the version of the pkg-config macros being
used. PKG_PREREQ (like AC_PREREQ or LT_PREREQ) can be used to check the
version of the macros in use when configure is being generated by the
developer.
https://bugs.freedesktop.org/show_bug.cgi?id=89267
The pkg-config(1) manual includes a lot of details behind pkg-config,
but not the background and usage patterns. This guide tries to provide
users and developers with a starting point for pkg-config.