Allow the user to specify CFLAGS without breaking the build

GNU make has a very interesting quirk: If you set a variable on the
command line, any changes to that variable in the Makefile are ignored
unless marked with the "override" keyword.

Use CFLAGS only for optimization and warning options, and use CPPFLAGS
for the dependency and other preprocessor related options.
That way packagers can specify their own CFLAGS without breaking the
build.

As a side benefit, the ich_descriptors_tool Makefile now behaves exactly
the same whether called standalone or as part of the main Makefile.

Corresponding to flashrom svn r1574.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
diff --git a/Makefile b/Makefile
index 774d5b3..c78f419 100644
--- a/Makefile
+++ b/Makefile
@@ -26,6 +26,9 @@
 # If you want to cross-compile, just run e.g.
 # make CC=i586-pc-msdosdjgpp-gcc
 # You may have to specify STRIP/AR/RANLIB as well.
+#
+# Note for anyone editing this Makefile: gnumake will happily ignore any
+# changes in this Makefile to variables set on the command line.
 CC      ?= gcc
 STRIP   ?= strip
 INSTALL = install
@@ -83,7 +86,7 @@
 EXEC_SUFFIX := .exe
 CPPFLAGS += -I../libgetopt
 # DJGPP has odd uint*_t definitions which cause lots of format string warnings.
-CPPFLAGS += -Wno-format
+CFLAGS += -Wno-format
 # FIXME Check if we can achieve the same effect with -L../libgetopt -lgetopt
 LIBS += ../libgetopt/libgetopt.a
 # Bus Pirate, Serprog and PonyProg are not supported under DOS (missing serial support).
@@ -119,7 +122,7 @@
 ifeq ($(TARGET_OS), MinGW)
 EXEC_SUFFIX := .exe
 # MinGW doesn't have the ffs() function, but we can use gcc's __builtin_ffs().
-CFLAGS += -Dffs=__builtin_ffs
+CPPFLAGS += -Dffs=__builtin_ffs
 # libusb-win32/libftdi stuff is usually installed in /usr/local.
 CPPFLAGS += -I/usr/local/include
 LDFLAGS += -L/usr/local/lib