https://bugzilla.redhat.com/show_bug.cgi?id=2149698
--- Comment #10 from Caleb Maclennan caleb@alerque.com --- Hey, upstream SILE and libtexpdf maintainer here.
Long term I 100% agree that getting the libtexpdf projects split out into its own project is the goal we want to achieve, but we're not there yet. We already have it as a separate Git repository because we knew we wanted to allow it to be independent, but it really isn't there yet. To date it has no stand-alone releases and the Makefile even makes assumptions about being a submodule of the SILE repository.
Until it has it's own releases and fully stand alone build system I would suggest just building with it being installed as part of the SILE package. As soon as it does have it's own tagged releases then a new package could provide the library and the SILE package could be changed to depend on it.