[petsc-dev] GCC8 Fortran length changes from int to size_t

[Jed Brown]

See Fortran Language Issues.

  https://gcc.gnu.org/gcc-8/porting_to.html

We'll have to test for this (probably compiler version) and change the
PETSC_MIXED_LEN / PETSC_END_LEN to use size_t instead of int.