* cleaned up oss-fuzz targets / use LIB_FUZZING_ENGINE for actual client
* fixed some compiler warnings in oss-fuzz sources
* only build the fuzz-client in Travis
* make fuzz-client CMake target work with CMake < 3
* fixed compilation of OSS-Fuzz clients
* added preliminary CMake target for fuzz-client - also added *_sanitized targets of dependencies (only available with Clang)
* added oss-fuzz build to Travis CI