Don't use __declspec (dllimport) for PGDLLEXPORT to reduce warnings

by gcc version 4 on mingw and cygwin. We don't use dllexport here
because dllexport and dllwrap don't work well together.

src/include/port/cygwin.h

@@ -19,4 +19,4 @@
 #define PGDLLIMPORT __declspec (dllimport)
 #endif
 
-#define PGDLLEXPORT __declspec (dllimport)
+#define PGDLLEXPORT

src/include/port/win32.h

@@ -61,7 +61,7 @@
 #ifdef _MSC_VER
 #define PGDLLEXPORT __declspec (dllexport)
 #else
-#define PGDLLEXPORT __declspec (dllimport)
+#define PGDLLEXPORT
 #endif
 #else							/* not CYGWIN, not MSVC, not MingW */
 #define PGDLLIMPORT