bug#35405: Guile stable-2.2 HEAD fails to compile on MingGW-W64.

[John Ralls <jralls@ceridwen.us>]

1. 
In file included from C:/gcdev64/gnucash/maint/src/guile/lib/poll.c:39:0:
C:/gcdev64/msys2/mingw32/i686-w64-mingw32/include/winsock2.h:1155:16: error: redefinition of 'struct pollfd'
 typedef struct pollfd {
                ^~~~~~
In file included from C:/gcdev64/gnucash/maint/src/guile/lib/poll.c:32:0:
./poll.h:405:8: note: originally defined here
 struct pollfd
        ^~~~~~
  CC       recvfrom.lo
C:/gcdev64/gnucash/maint/src/guile/lib/poll.c:329:1: error: conflicting types for 'poll'
 poll (struct pollfd *pfd, nfds_t nfd, int timeout)
 ^~~~
In file included from C:/gcdev64/gnucash/maint/src/guile/lib/poll.c:32:0:
./poll.h:435:1: note: previous declaration of 'poll' was here
 _GL_FUNCDECL_SYS (poll, int, (struct pollfd *pfd, nfds_t nfd, int timeout));
 ^

2.

C:/gcdev64/gnucash/maint/src/guile/libguile/socket.c:43:10: fatal error: netinet/tcp.h: No such file or directory
 #include <netinet/tcp.h>
          ^~~~~~~~~~~~~~~
3.
../lib/.libs/libgnu.a(timegm.o): In function `timegm':
C:/gcdev64/gnucash/maint/src/guile/lib/timegm.c:39: undefined reference to `mktime_internal'