As Fedora 26 has a new gcc [1] and a new system pkg-config implementation [2] I
wanted to do some testing on that host OS so that we might be able to get some
fixes in before Pyro/2.3 is released.

The following series were the only changes required to start building (I have
tested core-image-minimal, core-image-base and core-image-sato) on Fedora 26.

1. https://fedoraproject.org/wiki/Changes/GCC7
2. https://fedoraproject.org/wiki/Changes/pkgconf_as_system_pkg-config_implementation