fix static build and backend loading for mingw32

git-svn-id: https://hamlib.svn.sourceforge.net/svnroot/hamlib/trunk@2671 7ae35d74-ebe9-4afe-98af-79ac388436b8

include/hamlib/rig_dll.h

@@ -41,8 +41,8 @@
 #      define HAMLIB_IMPEXP HAMLIB_DLL_IMPORT
 #    endif
 #  else
-#      define HAMLIB_IMPEXP HAMLIB_DLL_IMPORT
-	 /* must be static build, no directive? */
+       /* static build, only export the backend entry points for lt_dlsym */
+#      define BACKEND_IMPEXP HAMLIB_DLL_EXPORT
 #  endif
 #endif