Revert "Check __ANDROID_API__ instead of defining it"

This reverts commit f934a0b.

BUILDING.md

@@ -58,9 +58,6 @@ Note in particular that if you are cross-compiling an x86 build on a 64-bit
 version of Linux, then you need to have the proper gcc-multilibs and
 g++-multilibs packages or equivalent installed.
 
-On Android, you will need to define the `__ANDROID_API__` constant to at least
-`21` on 64-bit and `18` on 32-bit.
-
 
 
 Additional Features that are Useful for Development

build.rs

@@ -589,6 +589,17 @@ fn cc(file: &Path, ext: &str, target: &Target, warnings_are_errors: bool,
         // http://www.openwall.com/lists/musl/2015/06/17/1
         let _ = c.flag("-U_FORTIFY_SOURCE");
     }
+    if target.os() == "android" {
+        // Define __ANDROID_API__ to the Android API level we want.
+        // Needed for Android NDK Unified Headers, see:
+        // https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md#Supporting-Unified-Headers-in-Your-Build-System
+        if target.arch() == "aarch64" {
+            // Minimum API level where AArch64 is available is 21.
+            let _ = c.define("__ANDROID_API__", Some("21"));
+        } else {
+            let _ = c.define("__ANDROID_API__", Some("18"));
+        }
+    }
 
     let mut c = c.get_compiler().to_command();
     let _ = c.arg("-c")