cmake: own the qemu-uae plugin provisioning

cmake: own the qemu-uae plugin provisioning

The mandatory qemu-uae.so plugin was only buildable from a sibling
qemu-uae source tree; distro builds without one configured fine, built
fine, and then failed at install time when the install rule referenced
the never-built plugin (GitHub issue #6). Both CI workflows carried
their own duplicated checkout/build/strip plumbing for the
uae-ppc-plugin builder instead.

Resolve the plugin at configure time, in order: prebuilt
WINUAE_QEMU_UAE_PLUGIN_FILE, the sibling qemu-uae source tree, an
uae-ppc-plugin builder checkout (WINUAE_QEMU_UAE_BUILDER_DIR, default
sibling; WINUAE_QEMU_UAE_QEMU_TARBALL feeds it a local QEMU tarball for
offline distribution builds), or a build-time clone of the builder
(WINUAE_QEMU_UAE_BUILDER_FETCH, default ON). When nothing applies the
configure fails with the list of options. On Linux the plugin target is
part of the default build so plain build+install flows always bundle
the plugin.

Drive the builder through an OUTPUT-based custom command so repeated
`all` builds and CPack preinstall do not rerun the external builder once
qemu-uae.so is already present.

The Ubuntu and macOS workflows drop their manual plugin build steps and
pass the builder checkout to CMake; stripping moved into the builder
wrapper script.