matplotlib: only use pkgconf if it's a dependency
We assume pkg-config
is provided by the host on Linux. We should only
set the PKG_CONFIG
environment variable if the dependency is actually
available.
Reported and debugged here: https://discourse.paraview.org/t/paraview-5-9-0-rc1-superbuild-package-configuration-issue-affecting-matplotlibs-detection-of-freetype-and-zlib/5831
Cc: @utkarsh.ayachit