Add ability to build on Android (with opt-in)

Author: bbqsrc

CHANGELOG.md

@@ -11,6 +11,7 @@ This version of `cargo-ndk` is much closer to a thin passthrough to `cargo` than
 - **Breaking change**: Removed Cargo.toml-based configuration property handling
 - **Breaking change**: Bump minimum supported Rust version (MSRV) to 1.86 and use Rust edition 2024.
 - **Breaking change**: `--bindgen` has been removed, and relevant environment variables are set by default
+- Enhancement: Can now be built for an Android host (i.e. Termux) if built with `CARGO_NDK_ON_ANDROID` environment variable set
 - Enhancement: CLI flags can now be used in any order (e.g., `cargo ndk -t x86 build` and `cargo ndk build --target x86` are equivalent).
 - Enhancement: Added environment variable support for configuration flags with `CARGO_NDK_` prefix (e.g., `CARGO_NDK_TARGET`, `CARGO_NDK_PLATFORM`, `CARGO_NDK_OUTPUT_DIR`)
 - Enhancement: Target flag now supports comma-delimited lists for specifying multiple targets

build.rs

@@ -0,0 +1,9 @@
+fn main() {
+    println!("cargo:rerun-if-env-changed=CARGO_NDK_ON_ANDROID");
+
+    if std::env::var("CARGO_NDK_ON_ANDROID").is_ok() {
+        println!("cargo:rustc-cfg=cargo_ndk_on_android");
+    }
+
+    println!("cargo:rustc-check-cfg=cfg(cargo_ndk_on_android)");
+}

src/cargo.rs

@@ -14,14 +14,18 @@ use crate::shell::Shell;
 
 #[cfg(target_os = "macos")]
 const ARCH: &str = "darwin-x86_64";
-#[cfg(target_os = "linux")]
+#[cfg(any(target_os = "linux", target_os = "android"))]
 const ARCH: &str = "linux-x86_64";
 #[cfg(target_os = "windows")]
 const ARCH: &str = "windows-x86_64";
 
-#[cfg(target_os = "android")]
+#[cfg(all(target_os = "android", not(cargo_ndk_on_android)))]
 compile_error!(
-    "You cannot build cargo-ndk _for_ Android. Build it for your host OS and run it with cargo."
+    r#"
+Building cargo-ndk on Android is not supported. This binary is intended to be run on your host OS.
+
+Set CARGO_NDK_ON_ANDROID to override this check (for example, building for Termux)."
+"#
 );
 
 #[cfg(not(any(
@@ -31,8 +35,6 @@ compile_error!(
     target_os = "windows"
 )))]
 compile_error!("Unsupported target OS");
-#[cfg(not(any(target_os = "macos", target_os = "linux", target_os = "windows")))]
-const ARCH: &str = "unknown";
 
 pub(crate) fn clang_target(rust_target: &str, api_level: u8) -> String {
     let target = match rust_target {