Log Message:
libcerf: updated to 1.13

libcerf-1.13:
  - Further adjustments for compilation under Windows

libcerf-1.12:
  - Require CMake 3.6, outcomment code that requires 3.13.
  - Relative paths in CMake sources, for use as subproject.
  - When compiling as CPP, then #include<complex>, not <complex.h>;
    revise the entire C-vs-CPP machinery.
  - Remove tests with different inf or nan results on different systems or under
    different compilers.