Set DEF_FILE & IMPORT_LIBRARY to NULL when setting SHARED_LIBRARY to NULL. Should fix OS/2 tinderbox bustage.

config/rules.mk

@@ -176,11 +176,15 @@ endif
 
 ifeq (,$(BUILD_SHARED_LIBS)$(FORCE_SHARED_LIB))
 SHARED_LIBRARY		:= $(NULL)
+DEF_FILE		:= $(NULL)
+IMPORT_LIBRARY		:= $(NULL)
 endif
 
 ifdef FORCE_STATIC_LIB
 ifndef FORCE_SHARED_LIB
 SHARED_LIBRARY		:= $(NULL)
+DEF_FILE		:= $(NULL)
+IMPORT_LIBRARY		:= $(NULL)
 endif
 endif