Add optional feature 'no_cfg_fuzzing'

This is to allow not setting 'fuzzing' flag when compiling fuzzed
targets.
Some crates (eg. secp256k1) stub some functions, when they detect
'fuzzing' flag, which makes them useless when fuzzing.

Cargo.toml

@@ -31,3 +31,4 @@ tempfile = "3.4"
 
 [features]
 reset_lazy_static = ["lazy_static"]
+no_cfg_fuzzing = []

src/bin/cargo-afl.rs

@@ -363,8 +363,7 @@ where
     // `-C codegen-units=1` is needed to work around link errors
     // https://github.com/rust-fuzz/afl.rs/pull/193#issuecomment-933550430
     let mut rustflags = format!(
-        "--cfg fuzzing \
-         -C debug-assertions \
+        "-C debug-assertions \
          -C overflow_checks \
          -C passes={passes} \
          -C codegen-units=1 \
@@ -375,6 +374,10 @@ where
          -C target-cpu=native "
     );
 
+    if cfg!(not(feature = "no_cfg_fuzzing")) {
+        rustflags.push_str("--cfg fuzzing ");
+    }
+
     if cfg!(target_os = "linux") {
         // work around https://github.com/rust-fuzz/afl.rs/issues/141 /
         // https://github.com/rust-lang/rust/issues/53945, can be removed once