diff --git a/src/makefiles/Makefile.cygwin b/src/makefiles/Makefile.cygwin
index bd83e5f723..bb2efed3ce 100644
--- a/src/makefiles/Makefile.cygwin
+++ b/src/makefiles/Makefile.cygwin
@@ -28,6 +28,10 @@ ifneq (,$(findstring src/common,$(subdir)))
 override CPPFLAGS+= -DBUILDING_DLL
 endif
 
+ifneq (,$(findstring src/port,$(subdir)))
+override CPPFLAGS+= -DBUILDING_DLL
+endif
+
 ifneq (,$(findstring timezone,$(subdir)))
 override CPPFLAGS+= -DBUILDING_DLL
 endif
diff --git a/src/makefiles/Makefile.win32 b/src/makefiles/Makefile.win32
index b18621b2f8..9cb84f2502 100644
--- a/src/makefiles/Makefile.win32
+++ b/src/makefiles/Makefile.win32
@@ -27,6 +27,10 @@ ifneq (,$(findstring src/common,$(subdir)))
 override CPPFLAGS+= -DBUILDING_DLL
 endif
 
+ifneq (,$(findstring src/port,$(subdir)))
+override CPPFLAGS+= -DBUILDING_DLL
+endif
+
 ifneq (,$(findstring timezone,$(subdir)))
 override CPPFLAGS+= -DBUILDING_DLL
 endif