configure: define posix source on cygwin

Some function definitions are missing without it,
for example sigaction.

Fixes ticket #5868

configure

@@ -4802,6 +4802,7 @@ case $target_os in
         objformat="win32"
         enable dos_paths
         enabled shared && ! enabled small && check_cmd $windres --version && enable gnu_windres
+        add_cppflags -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600
         ;;
     *-dos|freedos|opendos)
         network_extralibs="-lsocket"