Hello,
unfortunately there's going to be *another* soname bump to libxc. Libxc 5 replaced the ancient "Fortran '90" interface with the Fortran 2003 version based on iso_c_binding. However, this included *renaming* the Fortran 2003 module and functions, which is obviously inconvenient for downstream projects and has caused a lot of hassle.
This change is reverted in the upcoming libxc release, resulting in another soname bump, but a more backwards compatible API... It appears that the Fortran codes elk and exciting have not yet been compiled against libxc 5; the next release will be easier to compile against.