Fix windows line markers

[melven]

In our Windows setup, python sysconfig.get_platform() results in win-amd64. So we get the wrong path separator in generated source files (\ instead of /) which leads to compiler errors.

[melven]

Not sure if this could break the setup for other people if there are Windows Fortran compilers that actually want to have:

#line C:\Some\Path\to\file.pfunit

instead of

#line C:/Some/Path/to/file.pfunit

[tclune]

Ah - needs a line in the ChangeLog please.

[mathomp4]

@melven We'd like to release this soon, could you add an entry to the changelog?

[melven]

Updated changelog.

[tclune]

@melven Apologies - I've been distracted by other projects. If you think this PR is ready for windows, I'll go ahead and merge and do a release.

[melven]

It's ready!