Makefile: pass CPPFLAGS through to fllow customization

Without this patch there is no straight forward way to pass additional
CPPFLAGS at configure-time.  At TWW, everything non-vendor package is
installed to its own subdirectory, so we need the following to show
the preprocessor where the headers for the libraries we will link
later can be found:

	$SHELL ./configure \
	CPPFLAGS="-I${SB_VAR_CURL_INC}\
	 -I${SB_VAR_LIBEXPAT_INC}\
	 -I${SB_VAR_LIBZ_INC}\
	${CPPFLAGS+ $CPPFLAGS}" <<...>>

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -246,7 +246,7 @@ endif
 
 CFLAGS = -g -O2 -Wall
 LDFLAGS =
-ALL_CFLAGS = $(CFLAGS)
+ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 STRIP ?= strip
 

config.mak.in

@@ -3,6 +3,7 @@
 
 CC = @CC@
 CFLAGS = @CFLAGS@
+CPPFLAGS = @CPPFLAGS@
 LDFLAGS = @LDFLAGS@
 CC_LD_DYNPATH = @CC_LD_DYNPATH@
 AR = @AR@