libkcapi build fails with the error message:

pdfjadetex aborts at \begin{document} with:
! Undefined control sequence.
<argument> \c_sys_backend_str

The current kernel \document (texmf-dist/tex/latex/base/latex.ltx) runs
\@expl@sys@load@backend@@ during \begin{document} and that step loads the
output backend that defines \c_sys_backend_str. 

jadetex's copy omits this step, so \c_sys_backend_str is never created.

Starting in https://github.com/latex3/hyperref/commit/e6d9094 hyperref reads \c_sys_backend_str inside a begindocument hook.

Adding at line 1312 in texmf-dist/tex/jadetex/base/jadetex.ltx allow libkcapi build to finish successfully:
\ifdefined\@expl@sys@load@backend@@ \@expl@sys@load@backend@@ \fi

Reproducible: Always

Steps to Reproduce:
1. Try to build the latest libkcapi on Rawhide or Fedora 44
Actual Results:
Build fails with:
! Undefined control sequence.
<argument> \c_sys_backend_str

Expected Results:
Build succeeeds