build: use -std=c99 and -std=c++11 by default

build: use -std=c99 and -std=c++11 by default

Relying on the compiler default is problematic:

* New constructs may be introduced accidentally that break backwards
  compatibility.
* Older compilers may default to a more restricted standard than the
  codebase is actually developed with.

Explicitly select C99 (and C++11 for the fuzzing part).

Autotools does not want to default to C99, so set it explcitly during
CI.

Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>