make: Append instead of override CPPFLAGS and CFLAGS passed by user

Distribution build scripts commonly assume the CPPFLAGS, CFLAGS and
LDFLAGS passed via command line or environment to be appended to the
base set of flags defined in the Makefile. Follow this assumption
instead of overriding the already defined values.

Also treat these three variables entriely independently instead of
appending CPPFLAGS to CFLAGS.

Reported-by: @pali
Really-fixes: #17
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

Makefile

@@ -18,7 +18,6 @@ CC	= $(CROSS_COMPILE)gcc
 INSTALL	= install
 
 CPPFLAGS ?=
-CFLAGS	?= -W -Wall -O2 $(CPPFLAGS)
 LDFLAGS	?=
 
 ifeq ($(shell git rev-parse > /dev/null 2>&1; echo $$?), 0)
@@ -27,12 +26,14 @@ else
   GIT_VERSION =
 endif
 
-CFLAGS	+= -DVERSION_STRING=\"v$(VERSION)\" -DGIT_VERSION=\"$(GIT_VERSION)\"
-
+CFLAGS_MIN := -W -Wall -DVERSION_STRING=\"v$(VERSION)\" -DGIT_VERSION=\"$(GIT_VERSION)\"
 ifeq ($(DEBUG), 1)
-  CFLAGS += -g -DDEBUG
+  CFLAGS_MIN += -g -DDEBUG
 endif
 
+CFLAGS ?= -O2
+override CFLAGS := $(CFLAGS_MIN) $(CFLAGS)
+
 Q	?= @
 ifeq ($(Q),)
   CCQ	= $(CC)
@@ -57,10 +58,10 @@ $(Q_P): $(Q_OBJS)
 	$(LDQ) $(LDFLAGS) -o $@ $(Q_OBJS) $(Q_LIBS)
 
 %.o: %.c %.h
-	$(CCQ) $(CFLAGS) -o $@ -c $<
+	$(CCQ) $(CFLAGS) $(CPPFLAGS) -o $@ -c $<
 
 %.o: %.c
-	$(CCQ) $(CFLAGS) -o $@ -c $<
+	$(CCQ) $(CFLAGS) $(CPPFLAGS) -o $@ -c $<
 
 install_$(D_P): $(D_P)
 	@echo "  INSTALL $(D_P)"