Makefile.in: pass CPPFLAGS where appropriate
CPPFLAGS is a standard variable to pass additional
preprocessor-related parameters.
Debian build system uses that to add -D_FORTIFY_SOURCE=2 and then
checks the buildlogs to ensure the command line contained all the
fortification parameters (they use "-fstack-protector
--param=ssp-buffer-size=4" for CFLAGS).
Example blhc output before the fix:
CPPFLAGS missing (-D_FORTIFY_SOURCE=2): cc -D_GNU_SOURCE -Wall -I. -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall -c -o _load-static-exts.o _load-static-exts.c
Signed-off-by: Paul Fertser <fercerpav@gmail.com>