Fix sysroot for android ndk 14+ (#1207)

See:
https://android.googlesource.com/platform/ndk/+/ndk-release-r16/docs/UnifiedHeaders.md

Author: tobil4sk

toolchain/android-toolchain-gcc.xml

@@ -66,7 +66,12 @@
   <exe name="${EXEPREFIX}-g++" />
 
   <!-- These must appear in this order! -->
-  <flag value="--sysroot=${ANDROID_NDK_ROOT}/platforms/${PLATFORM}/${PLATFORM_ARCH}"/>
+  <flag value="--sysroot=${ANDROID_NDK_ROOT}/platforms/${PLATFORM}/${PLATFORM_ARCH}" unless="NDKV14+" />
+  <section if="NDKV14+">
+      <flag value="--sysroot=${ANDROID_NDK_ROOT}/sysroot" />
+      <flag name="-isystem" value="${ANDROID_NDK_ROOT}/sysroot/usr/include/${EXEPREFIX}" />
+      <flag value="-D__ANDROID_API__=${HXCPP_ANDROID_PLATFORM}" />
+  </section>
   <section if="NDKV8+">
     <flag value="-I${ANDROID_NDK_ROOT}/sources/cxx-stl/gnu-libstdc++/${TOOLCHAIN_VERSION}/include" />
     <flag value="-I${ANDROID_NDK_ROOT}/sources/cxx-stl/gnu-libstdc++/${TOOLCHAIN_VERSION}/libs/${ABI}/include" />