The sync-ui doesn't have a hard compile dependency on syncevo-dbus-server,
although it obviously won't run well without it. The new --disable-core
option can be used to compile just the sync-ui and the D-Bus wrapper lib
that it depends on.
Translations are generated when sync-ui and syncevo-dbus-server
are enabled, because both depend on the same translation files.
For the situation that sync-ui needs to include syerror.h from a
bundled Synthesis source, but libsynthesis itself is not built,
a special src/synthesis-includes directory is entered and produces
the necessary files by invoking the right target in the Synthesis
build dir. Because of that, enough packages must be installed to
configure the Synthesis engine even when only building the UI.
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