commit 459d202 (prefix special defines with FORTIFY_) changed
define. Adjust the testsuite accordingly.

Use `gcc` as default and pass the CC compiler with the github workflow
instead.

This makes it possible to do multiple runs without needing to clean in
between.

This allows us to disable warnings on Alpine Linux by doing:

   make -C tests FORTIFY_CFLAGS="-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3"

It also makes it possible to run the tests with -D_FORTIFY_SOURCE=2
without modifying the Makefile.

Set the system depended includes in github workflows, where we also
define the syste (ubuntu, x86_64 etc).

This makes it easier to run test suite on Alpine

- include a `config.mk` if it exists. This is useful to set local cflags
  while debugging. For example:
    echo "CFLAGS+=-U_FORTIFY_SOURCE" > config.mk
- Remove 'gcc' and 'clang' targets. Let user set that via CC.
- Add a `check` target, so user can run the test suite with
  `make check`.
- set -nostdinc always to avoid pull in system headers