Got an answer from the mailing list, by Claude Heiland-Allen:
Windows has a different notion of paths to unix-like systems, including drive specifiers.
The first line of moonlib/absolutepath.c[1] is
#ifndef _WIN32
so the file is empty when compiling for Windows. (Perhaps it could contain an alternative implementation that prints a more > useful error message.)
The non-Windows implementation uses dirname() from libgen.h, which isn't available on Windows as far as I can tell. A suggested alternative is _splitpath_s[2].
[1] http://sourceforge.net/p/pure-data/svn/HEAD/tree/trunk/externals/moonlib/absolutepath.c#l1
[2] https://msdn.microsoft.com/en-us/library/8e46eyt7.aspx
So in that case, these are not Windows compatible I am afraid...