savewd: please port to native Windows

[Bruno Haible]

clang on native Windows warns about an undefined function:

gllib\savewd.c(73,27): warning: implicit declaration of function 'fork' is 
invalid in C99 [-Wimplicit-function-declaration]

There is no 'fork' function on native Windows.

It's not clear to me whether this module can still partially work on native
Windows, with some #if. Or if it is better disabled entirely, by putting
all the code under
  #if !(defined _WIN32 && !defined __CYGWIN__)

Can someone please take a look?

Bruno