It fails on MinGW which needs more thorough examination on why and how.
Happy for the time being, since it compiles

Double backslash escape in config.h.in to get one single backslash escape
config.h. That will then be unescaped backslash in the code hopefully.

…in Windows builds.

The issue was the conflict between DATADIR macro and the Windows SDK's DATADIR
enum type defined in objidl.h.

By prefixing all the macros with GERBV_, we've avoided the namespace collision with
the Windows headers while maintaining all the functionality.

Using %ld for printing out ssize_t is not platform independent and
it became obvious when cross compiling using mingw.
Using unsigned int for number of lines should be enough and thus
using %u is platform independent.