Makefile: Use CPPFLAGS

Since commit 500c36c8 ("Makefile: Make it POSIX.1-2024 compatible"),
the implicit rule, which used CPPFLAGS, is replaced by a custom rule
that does not use it.

Restore the use of CPPFLAGS so that packaging systems can customize it,
by e.g. adding -D_FORTIFY_SOURCE=2