relates #722

relates #638

Implements initial support for continuous fuzzing with [google/oss-fuzz](http://github.com/google/oss-fuzz). Changes:

- Removes `--with-santize-coverage` config flag: the clang6 `-fsanitize=fuzzer-no-link` replaces this
- Adds `--with-oss-fuzz` config flag: links `-lFuzzingEngine` into fuzz targets
- Adds logic to `make check` that runs the fuzz targets with a single seed input to ensure they exit successfully

...except OSX where the compile option is not available

AM_EXTRA_RECURSIVE_TARGETS works since automake 1.13. (CentOS 6 has 1.11.)