With OSS-Fuzz, the build system should not directly set options such as
-fsanitize=fuzzer. Instead, these are set by OSS-Fuzz, and
linker options are provided via the LIB_FUZZER_OPTIONS environment
variable. This change allows the fuzzers to be build stand-alone,
outside of OSS-Fuzz, in the way that was already supported, as well as
inside OSS-Fuzz, when the LIB_FUZZER_OPTIONS environment variable is
set.