Link against dynamic C++ stdlib on Android by default.

It's a configurable feature that can be disabled if needed, but I got a
bunch of linker errors about unavailable C++ symbols from the static
c++.a for some reason.

Author: vlovich

llama-cpp-2/Cargo.toml

@@ -18,14 +18,16 @@ tracing = { workspace = true }
 encoding_rs = { workspace = true }
 
 [features]
-default = ["openmp"]
+default = ["openmp", "android-shared-stdcxx"]
 cuda = ["llama-cpp-sys-2/cuda"]
 metal = ["llama-cpp-sys-2/metal"]
 dynamic-link = ["llama-cpp-sys-2/dynamic-link"]
 vulkan = ["llama-cpp-sys-2/vulkan"]
 native = ["llama-cpp-sys-2/native"]
 openmp = ["llama-cpp-sys-2/openmp"]
 sampler = []
+# Only has an impact on Android.
+android-shared-stdcxx = ["llama-cpp-sys-2/shared-stdcxx"]
 
 
 [target.'cfg(all(target_os = "macos", any(target_arch = "aarch64", target_arch = "arm64")))'.dependencies]

llama-cpp-sys-2/Cargo.toml

@@ -71,3 +71,5 @@ dynamic-link = []
 vulkan = []
 native = []
 openmp = []
+# Only has an impact on Android.
+shared-stdcxx = []

llama-cpp-sys-2/build.rs

@@ -263,6 +263,10 @@ fn main() {
             panic!("Unsupported Android target {target}");
         }
         config.define("GGML_LLAMAFILE", "OFF");
+        if cfg!(feature = "shared-stdcxx") {
+            println!("cargo:rustc-link-lib=dylib=stdc++");
+            println!("cargo:rustc-link-lib=c++_shared");
+        }
     }
 
     if cfg!(feature = "vulkan") {