Use CPPFLAGS consistently

[solardiz]

We got some uses of CPPFLAGS introduced into Makefile along with i18n support. However, those are not for all targets. Apparently, Debian splits their hardening flags between CFLAGS and CPPFLAGS, and they have something spitting out warnings if some of the flags didn't get into some of the compiler invocations. In their package of passwdqc, they use a workaround:

override_dh_auto_build:
	dh_auto_build -- CFLAGS="-Wall -W $(CFLAGS) $(CPPFLAGS)" LDFLAGS="$(LDFLAGS)" LDFLAGS_shared_LINUX="--shared $(LDFLAGS)"

However, we should probably start supporting CPPFLAGS consistently, to let them drop this workaround. (And in other Openwall software releases, too.)

@ldv-alt What do you think of this? If you agree, would you please create a pull request completing the CPPFLAGS support?