Absence of '__attribute__(cleanup)' would lead to memory leaks.

The change didn't work as expected :-/

This reverts commit d71091d1.

It's needed for Debian Stretch and Ubuntu Xenial

gnutls 3.6.12 adds support for this, given that you use sufficiently new
nettle (not released yet).  I tested this with Knot Resolver and
https://rootcanary.org/test.html

Note: in some switch statements I moved the labels inside #ifdef guards.
Otherwise the fall-through wouldn't be correct in the weird case of
not supporting ed25519 but supporting ed448 (shouldn't happen anyway).

It's not particularly useful to record -fdebug-prefix-map= and friends
in the list of used CFLAGS arguments.  Additionally, since these
compiler arguments are typically used to make reproducible binary
artifacts by stripping out information about the ephemeral build path
used, embedding them in the resulting binary actually re-introduces
the reproducibility problem.

Note that gcc itself omits these values from the DWARF information
that it generates:

https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/dwarf2out.c;h=1b17f2bc1d5eef3d3ef70abf92a4d09da285d2b8;hb=HEAD#l24396

You can see the reproducibility issue here:

    https://tests.reproducible-builds.org/debian/dbd/unstable/amd64/knot_2.7.6-2.diffoscope.html