Fix compilation with custom CFLAGS

With this small change it is possible to build flashrom again when
specifying custom CFLAGS/LDFLAGS from the make command line like.

  make CFLAGS="..." LDFLAGS="..."

I need to do this when building flashrom in a cross compiler environment
like buildroot for a foreign target.

Corresponding to flashrom svn r196 and coreboot v2 svn r3102.
Signed-off-by: Clark Rawlins <clark@bit63.org>
Acked-by: Ronald G. Minnich <rminnich@gmail.com>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>

Makefile

@@ -32,12 +32,14 @@ all: pciutils dep $(PROGRAM)
 # of the checked out flashrom files.
 SVNDEF := -D'FLASHROM_VERSION="$(shell svnversion -cn . \
           | sed -e "s/.*://" -e "s/\([0-9]*\).*/\1/")"'
-CFLAGS += $(SVNDEF)
 
 $(PROGRAM): $(OBJS)
 	$(CC) -o $(PROGRAM) $(OBJS) $(LDFLAGS)
 	$(STRIP) $(STRIP_ARGS) $(PROGRAM)
 
+flashrom.o: flashrom.c
+	$(CC) -c $(CFLAGS) $(SVNDEF) $(CPPFLAGS) $< -o $@
+
 clean:
 	rm -f *.o *~