chore: add Cargo features

Cargo.toml

@@ -25,10 +25,16 @@ repository.workspace = true
 rust-version.workspace = true
 
 [dependencies]
-nginx-sys = { path = "nginx-sys", version = "0.5.0"}
+nginx-sys = { path = "nginx-sys", default-features=false, version = "0.5.0"}
 
 [features]
-default = ["vendored"]
+default = ["vendored","std"]
+# Enables the components using memory allocation.
+# If no `std` flag, `alloc` crate is internally used instead. This flag is mainly for `no_std` build.
+alloc = []
+# Enables the components using `std` crate.
+# Currently the only difference to `alloc` flag is `std::error::Error` implementation.
+std = ["alloc"]
 # Build our own copy of the NGINX by default.
 # This could be disabled with `--no-default-features` to minimize the dependency tree
 # when building against an existing copy of the NGINX with the NGX_OBJS variable.

examples/Cargo.toml

@@ -12,7 +12,7 @@ build = "../build.rs"
 
 [dependencies]
 nginx-sys = { path = "../nginx-sys/", default-features = false }
-ngx = { path = "../", default-features = false }
+ngx = { path = "../", default-features = false, features = ["std"] }
 
 [dev-dependencies]
 aws-sign-v4 = "0.3.0"