Consistently define BUILDING_DLL during builds of src/port for Windows.

The MSVC build process already did so; this fixes the principal build
process to match.  Both processes already did likewise for src/common.
This lets server builds of src/port reference postgres.exe data symbols.

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

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