gitlab-ci.d: Build with --enable-fdt=system by default

gitlab-ci.d: Build with --enable-fdt=system by default

By using --enable-fdt=system we can make sure that the configure
script does not try to check out the "dtc" submodule. This should
help to safe some precious CI minutes in the long run.

While we're at it, also drop some now-redundant --enable-slirp
and --enable-capstone statements. These used to have the "=system"
suffix in the past, too, which has been dropped when the their
corresponding submodules had been removed. Since these features
are auto-enabled anyway now (since the containers have the right
libraries installed), we do not need the explicit --enable-...
statements anymore.

Message-Id: <20230207201447.566661-6-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>