Support building on Android

* Remove `-stdlib=libstdc++` flag on Android since it breaks the build
* Add `bundled-android-static-libstdcpp` feature for selecting the static C++ runtime (I.e. https://developer.android.com/ndk/guides/cpp-support#use_clang_directly)

crates/duckdb/Cargo.toml

@@ -20,6 +20,7 @@ name = "duckdb"
 [features]
 default = []
 bundled = ["libduckdb-sys/bundled"]
+bundled-android-static-libstdcpp = ["libduckdb-sys/bundled-android-static-libstdcpp"]
 json = ["libduckdb-sys/json", "bundled"]
 parquet = ["libduckdb-sys/parquet", "bundled"]
 vtab = []

crates/libduckdb-sys/Cargo.toml

@@ -18,6 +18,7 @@ exclude = ["duckdb-sources"]
 [features]
 default = ["vcpkg", "pkg-config"]
 bundled = ["cc"]
+bundled-android-static-libstdcpp = ["bundled"]
 buildtime_bindgen = ["bindgen", "pkg-config", "vcpkg"]
 json = ["bundled"]
 parquet = ["bundled"]

crates/libduckdb-sys/build.rs

@@ -37,6 +37,7 @@ fn main() {
 mod build_bundled {
     use std::{
         collections::{HashMap, HashSet},
+        env,
         path::Path,
     };
 
@@ -151,11 +152,20 @@ mod build_bundled {
         cfg.cpp(true)
             .flag_if_supported("-std=c++11")
             .flag_if_supported("-stdlib=libc++")
-            .flag_if_supported("-stdlib=libstdc++")
             .flag_if_supported("/bigobj")
             .warnings(false)
             .flag_if_supported("-w");
 
+        // The Android NDK doesn't build with this flag set.
+        if env::var("CARGO_CFG_TARGET_OS").unwrap() != "android" {
+            cfg.flag_if_supported("-stdlib=libstdc++");
+        }
+
+        #[cfg(feature = "bundled-android-static-libstdcpp")]
+        if env::var("CARGO_CFG_TARGET_OS").unwrap() == "android" {
+            cfg.flag_if_supported("-static-libstdc++");
+        }
+
         if win_target() {
             cfg.define("DUCKDB_BUILD_LIBRARY", None);
         }