configure: warn at the end if pkg-config isn't available
configure: warn at the end if pkg-config isn't available

pciaccess is checked with pkg-config and manually, no problem.
libxml2 is only checked with pkg-config, but most people don't care.
Cairo is only checked with pkg-config, that's the main reason for not
getting graphical lstopo output.

shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc