Add weak symbol for forkserver (#3196)

* Add weak global symbol sharedmem_fuzzing

* Try

* Try

* Win

libafl_targets/src/coverage.c

@@ -14,6 +14,12 @@ uint8_t       *__afl_area_ptr = __afl_area_ptr_local;
 extern uint32_t __afl_acc_memop_ptr_local[ACCOUNTING_MAP_SIZE];
 uint32_t       *__afl_acc_memop_ptr = __afl_acc_memop_ptr_local;
 
+// Set by this macro
+// https://github.com/AFLplusplus/AFLplusplus/blob/stable/src/afl-cc.c#L993
+#if !defined(_WIN32)
+EXT_VAR(__afl_sharedmem_fuzzing, int);
+#endif
+
 // Weak symbols, LLVM Passes overwrites them if we really use it
 #if defined(__linux__)
 extern EXT_VAR(__start_libafl_token, uint8_t);