How to patch fixincludes/Makefile for Interix?

[Douglas B Rupp]

I've been working on a resurrection patch for Interix, one remaining 
problem is fixincludes/fixincl.c must be compiled on native Interix with 
-D_ALL_SOURCE in order to find the getpagesize prototype.

This works (taking some liberties with diff...):

fixincludes/Makefile.in
-FIXINC_CFLAGS = -DHAVE_CONFIG_H $(INCLUDES)
+FIXINC_CFLAGS = -DHAVE_CONFIG_H $(INCLUDES) -D_ALL_SOURCE

Obviously unacceptable in general.
Fixincludes ignores a config/mh-interix, so adding
FIXINC_CFLAGS += -D_ALL_SOURCE doesn't work.

What is the correct solution? Any advice would be greatly appreciated.