docs: Allow for .exe suffix in pandoc version detection regexp.
docs: Add make_doc --pandoc option and use it.

The build system can now cause make_doc to use the detected path to pandoc
using said option. Previously it was intended to do so by setting the
environment variable PANDOC, but that is not trivial in a CMake custom
command so it was left and forgotten about.
docs: Delete Windows path workaround for older versions of Pandoc.
docs: Use tempnam instead of tmpnam in make_doc.

tmpnam returns a path with prefix "/" on MSVC.  As we naively open a
file at the returned path directly, we were creating temporary files in
the root directory.  That is already not ideal but, worse, we may also
lack write permission there.
Prepare summary of changes for 5.1.8.
Bump version to 5.1.8 (WIP).
Bump version to 5.1.9 (GIT).