coturn fails to cross build from source, because it uses the build
architecture pkg-config and fails location dependencies as a result. It
has a hand-written configure script that somehow manages to figure out
the correct CC, but it uses the non-standard PKGCONFIG variable for
pkg-config when much of the rest of the world uses PKG_CONFIG.
Reexporting it under the other name fixes the cross build. Please
consider applying the attached patch.

Helmut