Only set MAPFILE if MOZ_MAPINFO is set - win32 gmake build

Bug #129161 r=walk84 a=asa

config/config.mk

@@ -312,8 +312,8 @@ endif
 # NS_TRACE_MALLOC
 
 # if MOZ_DEBUG is not set and MOZ_MAPINFO
-MAPFILE=$(LIBRARY_NAME).map
 ifdef MOZ_MAPINFO
+MAPFILE=$(LIBRARY_NAME).map
 OS_LDFLAGS += /MAP:$(MAPFILE) /MAPINFO:LINES
 endif
 #MOZ_MAPINFO