Fix cross-compilation by passing CPPFLAGS to CPP

src/Makefile.am

@@ -108,7 +108,7 @@ fcobjshash.gperf: Makefile stamp-fcobjshash.gperf
 	@$(MAKE) stamp-fcobjshash.gperf
 	@touch -r stamp-fcobjshash.gperf $@
 stamp-fcobjshash.gperf: fcobjshash.gperf.h fcobjs.h
-	$(AM_V_GEN) $(CPP) -I$(top_srcdir) $< | \
+	$(AM_V_GEN) $(CPP) -I$(top_srcdir) $(CPPFLAGS) $< | \
 	$(SED) 's/^ *//;s/ *, */,/' | \
 	$(GREP) '^[^#]' | \
 	awk ' \