src/port/dirmod.c needs to be built on Cygwin too.

Oversight in my commit 91f4a5a976.
Per buildfarm member brolga.

configure.in

@@ -1436,7 +1436,7 @@ if test "$PORTNAME" = "win32"; then
   AC_LIBOBJ(getopt_long)
 fi
 
-# Win32 support
+# Win32 (really MinGW) support
 if test "$PORTNAME" = "win32"; then
   AC_REPLACE_FUNCS(gettimeofday)
   AC_LIBOBJ(dirmod)
@@ -1460,6 +1460,11 @@ else
   AC_SUBST(have_win32_dbghelp,no)
 fi
 
+# Cygwin needs only a bit of that
+if test "$PORTNAME" = "cygwin"; then
+  AC_LIBOBJ(dirmod)
+fi
+
 dnl Cannot use AC_CHECK_FUNC because sigsetjmp may be a macro
 dnl (especially on GNU libc)
 dnl See also comments in c.h.