flatpak: Explicitly specify the LibRaw build options
Currently the build detects the presence of the necessary dependencies and automatically enables or disables the corresponding options. Unless someone is keeping a close eye on the build logs, a change in the manifest or the SDK might silently and unexpectedly change the build options.
Ideally, the build would fail if a requested option can't be met, to draw attention to this unforeseen change.
Unfortunately, the LibRaw build uses AC_MSG_WARN even for explicitly enabled build options. This means that a missing dependency won't fail the build even if the option was explicitly requested. While this is not ideal, at least listing the options makes it clear what the required outcome is. That way someone can manually check the logs from time to time and see if it's living up to the requirements.
The set of build flags have been chosen to match the current reality of automatically selected options. Hence, there shouldn't be any user-visible change.