What do I need to do to build (parts of) WebRTC for Android (NDK)? It can't find C++ headers

I'm trying to build some features of WebRTC into a project. We had previously gotten some C-only code functioning, and I'm trying to add more files. The new files are C++, and when I run ndk-build I'm getting errors related to the C++ includes:

[armeabi] Compile++ thumb: zello.webrtc <= audio_frame.cc
In file included from libwebrtc/jni/libwebrtc/api/audio/audio_frame.cc:15:
libwebrtc/jni/libwebrtc/rtc_base/checks.h:36:10: fatal error: 
  'sstream' file not found
#include <sstream>
         ^~~~~~~~~
1 error generated.

My Application.mk files includes

APP_ABI := armeabi armeabi-v7a arm64-v8a x86 x86_64
APP_OPTIM := release
APP_PLATFORM := android-15
APP_STL := c++_shared
APP_CPPFLAGS := -fexceptions
NDK_TOOLCHAIN_VERSION := clang

and my Android.mk file includes

LOCAL_CPPFLAGS := -std=c++11
LOCAL_CPP_EXTENSION := .cpp .cc
LOCAL_SRC_FILES := \
    webrtc-jni.cpp \
    libwebrtc/api/audio/audio_frame.cc \
    ...

I've tried explicitly adding

LOCAL_C_INCLUDES += $(NDK_ROOT)/sources/cxx-stl/llvm-libc++/include

but that just leads to more errors about undefined functions, and doesn't seem like it should be necessary in the first place. I've also tried specifying all the other values for APP_STL with no effect.

I am building with NDK 16b, since we still need to support armeabi devices.

Thanks in advance for any suggestions.

EDIT: When I run ndk-build V=1 I see that ndk-build is probably using the wrong STL header directory:

... -I${NDK_ROOT}/sources/cxx-stl/system/include ...

but I've defined APP_STL as c++_shared, which I think should be looking in a different directory.

Solution

The solution was to put the APP_STL definition in the top-level Application.mk file. Our app is structured like

top/Application.mk
top/Android.mk
top/...
top/submodule1/jni/Application.mk
top/submodule1/jni/Android.mk
top/submodule1/jni/...
top/submodule2/jni/Application.mk
top/submodule2/jni/Android.mk
top/submodule2/jni/...
...

I'd defined APP_STL in one of the submodule Application.mk files, which may have been entirely ignored? Moving the definition to the top level file got the build working correctly.