When targeting Android NDK 23+, use unwind library instead of gcc library. Fixes #75.

This works around an unfortunate issue with the Rust `std` library
where-by `-lgcc` is requested but not provided by newer NDKs.  There
are a multiple ways to address this, but since `rust-android-gradle`
already has a linker script it's reasonable to use it for this
purpose.

Author: minyung

plugin/src/main/kotlin/com/nishtahir/CargoBuildTask.kt

@@ -167,6 +167,10 @@ open class CargoBuildTask : DefaultTask() {
                 if (toolchain.type != ToolchainType.DESKTOP) {
                     val toolchainDirectory = if (toolchain.type == ToolchainType.ANDROID_PREBUILT) {
                         val ndkPath = app.ndkDirectory
+                        val ndkVersion = ndkPath.name
+                        val ndkVersionMajor = ndkVersion.split(".").first()
+                        environment("CARGO_NDK_MAJOR_VERSION", ndkVersionMajor)
+
                         val hostTag = if (Os.isFamily(Os.FAMILY_WINDOWS)) {
                             if (Os.isArch("x86_64") || Os.isArch("amd64")) {
                                 "windows-x86_64"

plugin/src/main/resources/com/nishtahir/linker-wrapper.py

@@ -7,6 +7,15 @@
 
 args = [os.environ['RUST_ANDROID_GRADLE_CC'], os.environ['RUST_ANDROID_GRADLE_CC_LINK_ARG']] + sys.argv[1:]
 
+# The `gcc` library is not included starting from NDK version 23.
+# Work around by using `unwind` replacement.
+ndk_major_version = os.environ['CARGO_NDK_MAJOR_VERSION']
+if ndk_major_version.isdigit():
+    if 23 <= int(ndk_major_version):
+        for i, arg in enumerate(args):
+            if arg == "-lgcc":
+                args[i] = "-lunwind"
+
 # This only appears when the subprocess call fails, but it's helpful then.
 printable_cmd = ' '.join(pipes.quote(arg) for arg in args)
 print(printable_cmd)