use same -I style as converter/other/Makefile so people can disable these

--- a/converter/other/jbig/Makefile
+++ b/converter/other/jbig/Makefile
@@ -11,7 +11,9 @@ LIBJBIG_OBJECTS = jbig.o jbig_tab.o
 
 EXTERN_INCLUDES =
 ifneq ($(JBIGHDR_DIR),NONE)
-  EXTERN_INCLUDES += -I$(JBIGHDR_DIR)
+  ifneq ($(JBIGHDR_DIR)x,x)
+    EXTERN_INCLUDES += -I$(JBIGHDR_DIR)
+  endif
 endif
 
 ifneq ($(JBIGHDR_DIR),NONE)
--- a/converter/other/jpeg2000/Makefile
+++ b/converter/other/jpeg2000/Makefile
@@ -11,7 +11,9 @@ include $(BUILDDIR)/config.mk
 
 EXTERN_INCLUDES =
 ifneq ($(JASPERHDR_DIR),NONE)
-  EXTERN_INCLUDES += -I$(JASPERHDR_DIR)
+  ifneq ($(JASPERHDR_DIR)x,x)
+    EXTERN_INCLUDES += -I$(JASPERHDR_DIR)
+  endif
 endif