community/buildah: re-enable on ppc64le & fix implicit declaration of basename

ppc64le was disabled in !61262 (merged), but it is actually ginkgo that is failing to build, not buildah.

make docs depends on make install.tools, and will build ginkgo, go-md2man, and golangci-lint.

Calling make -C docs directly prevents that, and passing the GOMD2MAN variable allows it to use the system go-md2man binary.