Use ndk metadata for default android platform

Instead of hardcoding default values per ndk version, it is better to
read the ndk metadata to figure out the minimum version supported by the
ndk.

Author: tobil4sk

toolchain/android-toolchain-clang.xml

@@ -1,12 +1,5 @@
 <xml>
 
-<section unless="HXCPP_ANDROID_PLATFORM" >
-  <set name="HXCPP_ANDROID_PLATFORM" value="16" />
-  <set name="HXCPP_ANDROID_PLATFORM" value="19" if="NDKV24+" />
-  <!-- 64 bit builds fail unless using platform 21 -->
-  <set name="HXCPP_ANDROID_PLATFORM" value="21" if="NDKV26+||HXCPP_X86_64||HXCPP_ARM64" />
-</section>
-
 <!-- Set architecture -->
 <section if="HXCPP_X86">
   <set name="ARCH" value="-x86" />

tools/hxcpp/Setup.hx

@@ -523,6 +523,28 @@ class Setup
          if (defines.exists("PLATFORM_NUMBER")) {
             Log.warn("The PLATFORM_NUMBER define is deprecated. Please use the HXCPP_ANDROID_PLATFORM define instead.");
             defines.set("HXCPP_ANDROID_PLATFORM", Std.string(defines.get("PLATFORM_NUMBER")));
+         } else {
+            var platformsJson = root + "/meta/platforms.json";
+
+            var minPlatform:Null<Int> = try {
+               haxe.Json.parse(sys.io.File.getContent(platformsJson)).min;
+            } catch (e) {
+               Log.warn("Unable to determine minimum supported Android platform: " + e.toString());
+               null;
+            };
+
+            if (minPlatform == null) {
+               Log.warn("Defaulting to Android platform 21");
+               minPlatform = 21;
+            }
+
+            // only platform version 21 and above support 64 bit
+            // https://developer.android.com/about/versions/lollipop#Perf
+            if (minPlatform < 21 && (defines.exists('HXCPP_ARM64') || defines.exists('HXCPP_X86_64'))) {
+               minPlatform = 21;
+            }
+
+            defines.set("HXCPP_ANDROID_PLATFORM", Std.string(minPlatform));
          }
       }
       else {