add support for multiple FUZZ_LIB arguments in oss-fuzz builds

FUZZER_LIB currently uses only "/libAFL.a" in fuzzer.py

Some fuzzers, such as KLEE, need to add other (non-static) libraries, see https://github.com/google/fuzzbench/blob/klee/fuzzers/klee/fuzzer.py#L43

However, oss-fuzz projects don't work with multiple arguments:

meson: error: unrecognized argument: -L -lKleeMock

meson is called with "-Dwith-fuzzer-lib=/libAFL.a -L/ -lKleeMock".

Maybe there is a way to have "-Dwith-fuzzer-lib=/libAFL.a -Dwith-fuzzer-lib=-lKleeMock -Dwith-some_option=-L"?