fuzz: better oss-fuzz integration

fuzz: better oss-fuzz integration

There were a few issues with the oss-fuzz integration from commit
8373f098542ecba3e0c58537f03760ea30c64ac2 ("build-sys: add oss-fuzz
support").

When building on OSS-Fuzz, the projects should use the provided CFLAGS
and CXXFLAGS and don't append any extra sanitization / fuzzing flags.

$LIB_FUZZING_ENGINE is defined to set the library to link to, and it
is a c++ library, so we should build fuzzer with c++...

Now --enable-fuzzer is only used for -fsanitize=fuzzer.

Add a tests/fuzz-main.c as fallback, to run the corpus on other builds.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>