I've recently uploaded libcgns 8.4.0-1~exp1 to experimental. All builds but ppc64el are ok.
The ppc64el failure [0] occurs in the test suite (test cgread_f03), when a C va_arg function (src/cg_ftoc.c:cg_goto_f()) is called from Fortran code (src/tests/cgread_f03.F90:421). This function is called several times. Is is successful at first, then it fails badly because the corresponding hidden string length parameters[1] are equal to 0. This shouldn't happen because the related strings are literals: 'Zone_t', 'GridCoordinates_t', 'end'.
[0]https://buildd.debian.org/status/fetch.php?pkg=libcgns&arch=ppc64el&ver=3.4.0-1%7Eexp1&stamp=1572269974&raw=0
[1]https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html
Further investigation shows that release 3.3.0-6 of libcgns FTBFS the very same way with GCC 8.3.0, while it succeeded with GCC 8.2.0 [2].
[2]https://buildd.debian.org/status/fetch.php?pkg=libcgns&arch=ppc64el&ver=3.3.0-6%2Bb2&stamp=1542797186&raw=0
I then ran a bisect on the GCC svn branch 'gcc-8-branch' and found out that the failure was introduced by the r269349 changeset [3].
[3]https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=269349
This is a backport of the trunk r268992 changetset [4] introduced during GCC-9 development.
[4]https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=268992