Makefile: Define CFLAGS_FORCE_C_SOURCE macro

This macro holds the C compiler flag(s) to force input files to be
recognized as C sources regardless of extension.  It is defined to allow
overriding on the make command line.
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -64,7 +64,7 @@ summary-fastcheck/%: tools/ccanlint/ccanlint $(OBJFILES)
 	$(CCANLINT_FAST) -s ccan/$*
 
 ccan/%/info: ccan/%/_info config.h
-	$(CC) $(CCAN_CFLAGS) -I. -o $@ -x c $<
+	$(CC) $(CCAN_CFLAGS) -I. -o $@ $(CFLAGS_FORCE_C_SOURCE) $<
 
 all_info: $(MODS:%=ccan/%/info)
 

Makefile-ccan

@@ -5,6 +5,7 @@
 #CCAN_CFLAGS=-g -O3 -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -Wundef -DCCAN_STR_DEBUG=1
 CCAN_CFLAGS=-g3 -ggdb -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -Wundef -DCCAN_STR_DEBUG=1
 CFLAGS = $(CCAN_CFLAGS) -I. $(DEPGEN)
+CFLAGS_FORCE_C_SOURCE = -x c
 
 MODS := a_star \
 	aga \