Weak linking is not working on MacOS

[tokatoka]

This line does not work on MacOS: https://github.com/AFLplusplus/LibAFL/blob/7d7601204f501959cefa3654e5e2ef82b44a3584/libafl_targets/src/common.h#L125 As a result, libafl_main in the lib is not marked as a weak symbols. https://github.com/AFLplusplus/LibAFL/blob/54ac57b6f736d6302a2c4399ffc8af563100dff1/libafl_targets/src/libfuzzer.c#L21 and user's libafl_main implementation is sometimes not picked up when compiling