GN: Android Vulkan support

gn/BUILDCONFIG.gn

Index: gn/BUILDCONFIG.gn
diff --git a/gn/BUILDCONFIG.gn b/gn/BUILDCONFIG.gn
index 1ef121a823b6c3cf4a1016ded11c236fe7b0295c..e6a8b695ffe29f5a363baed6330ead29674b0646 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -9,6 +9,7 @@ declare_args() {
   is_debug = true
   is_component_build = false
   ndk = ""
+  ndk_api = 0  # 0 == picked automatically for target_cpu.
   sanitize = ""
 }
 
@@ -45,7 +46,15 @@ if (is_android) {
   ndk_target = ""
   ndk_platform = ""
   ndk_stdlib = ""
-  nkd_gccdir = ""
+  ndk_gccdir = ""
+
+  if (ndk_api == 0) {
+    ndk_api = 18
+    if (target_cpu == "arm64" || target_cpu == "mips64el" ||
+        target_cpu == "x64") {
+      ndk_api = 21
+    }
+  }
 
   if (host_os == "linux") {
     ndk_host = "linux-x86_64"
@@ -55,32 +64,32 @@ if (is_android) {
 
   if (target_cpu == "arm64") {
     ndk_target = "aarch64-linux-android"
-    ndk_platform = "android-21/arch-arm64"
+    ndk_platform = "android-${ndk_api}/arch-arm64"
     ndk_stdlib = "arm64-v8a"
     ndk_gccdir = ndk_target
   } else if (target_cpu == "arm") {
     ndk_target = "arm-linux-androideabi"
-    ndk_platform = "android-18/arch-arm"
+    ndk_platform = "android-${ndk_api}/arch-arm"
     ndk_stdlib = "armeabi-v7a"
     ndk_gccdir = ndk_target
   } else if (target_cpu == "mips64el") {
     ndk_target = "mips64el-linux-android"
-    ndk_platform = "android-21/arch-mips64"
+    ndk_platform = "android-${ndk_api}/arch-mips64"
     ndk_stdlib = "mips64"
     ndk_gccdir = ndk_target
   } else if (target_cpu == "mipsel") {
     ndk_target = "mipsel-linux-android"
-    ndk_platform = "android-18/arch-mips"
+    ndk_platform = "android-${ndk_api}/arch-mips"
     ndk_stdlib = "mips"
     ndk_gccdir = ndk_target
   } else if (target_cpu == "x64") {
     ndk_target = "x86_64-linux-android"
-    ndk_platform = "android-21/arch-x86_64"
+    ndk_platform = "android-${ndk_api}/arch-x86_64"
     ndk_stdlib = "x86_64"
     ndk_gccdir = ndk_stdlib
   } else if (target_cpu == "x86") {
     ndk_target = "i686-linux-android"
-    ndk_platform = "android-18/arch-x86"
+    ndk_platform = "android-${ndk_api}/arch-x86"
     ndk_stdlib = "x86"
     ndk_gccdir = ndk_stdlib
   }