This variable is a standard way to inject options for the C
preprocessor. It's respected by the implicit rules of make
and autoconf/automake.
Debian sets this variable during package build to inject
`-D_FORTIFY_SOURCE=2` and we would like afl++ to respect it.
Note that this commit also adds $(CFLAGS) in the build of
afl-performance.o where it was missing. It might have been
on purpose but we want to keep CFLAGS everywhere as well
since Debian injects various options through that variable
(for hardening and reproducibility).
