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>