from the build log when trying to build win32 binaries from linux:

i686-w64-mingw32-gcc -DHAVE_CONFIG_H -I. -I../../common -I..
-DLOCALEDIR=\"/usr/i686-w64-mingw32/share/locale\"
-I/usr/i686-w64-mingw32/include -I/usr/i686-w64-mingw32/include
-I/usr/i686-w64-mingw32/include -I/usr/i686-w64-mingw32/include -DWITHOUT_NPTH=1
-Wall -Wno-pointer-sign -Wpointer-arith -g -Os -MT libcommon_a-w32-afunix.o -MD
-MP -MF .deps/libcommon_a-w32-afunix.Tpo -c -o libcommon_a-w32-afunix.o `test -f
'w32-afunix.c' || echo '../../common/'`w32-afunix.c
In file included from ../../common/w32-afunix.c:44:0:
../../common/w32-afunix.h:41:2: warning: #warning Remove this code; it is only
used on w32 by symcryptrun. [-Wcpp]
#warning Remove this code; it is only used on w32 by symcryptrun.

^

../../common/w32-afunix.c: In function ‘read_port_and_nonce’:
../../common/w32-afunix.c:65:7: warning: implicit declaration of function
‘gpg_err_set_errno’ [-Wimplicit-function-declaration]

gpg_err_set_errno (EIO);
^

mv -f .deps/libcommon_a-w32-afunix.Tpo .deps/libcommon_a-w32-afunix.Po

This could probably be fixed with the appropriate #include.