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

Oversight in my commit 91f4a5a9.
Per buildfarm member brolga.

configure

@@ -11973,7 +11973,7 @@ esac
 
 fi
 
-# Win32 support
+# Win32 (really MinGW) support
 if test "$PORTNAME" = "win32"; then
   ac_fn_c_check_func "$LINENO" "gettimeofday" "ac_cv_func_gettimeofday"
 if test "x$ac_cv_func_gettimeofday" = xyes; then :
@@ -12060,6 +12060,16 @@ else
 
 fi
 
+# Cygwin needs only a bit of that
+if test "$PORTNAME" = "cygwin"; then
+  case " $LIBOBJS " in
+  *" dirmod.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS dirmod.$ac_objext"
+ ;;
+esac
+
+fi
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for sigsetjmp" >&5
 $as_echo_n "checking for sigsetjmp... " >&6; }
 if ${pgac_cv_func_sigsetjmp+:} false; then :

configure.in

@@ -1451,7 +1451,7 @@ 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)
@@ -1475,6 +1475,11 @@ 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.