Fix build with glib 2.68 (Closes: #986969)
The newer glib 2.68 complicates the available usage of the extern "C" blocks. For more info: https://gitlab.gnome.org/GNOME/glib/-/issues/2331
Fedora has the fix: https://src.fedoraproject.org/rpms/gnucash/c/7287f9ed18964fac061035ba3a86566e1a03c4c5?branch=rawhide