gallium/auxiliary: Fix Autotools on Android (v2)

gallium/auxiliary: Fix Autotools on Android (v2)

Problem 1: u_debug_stack_android.cpp transitively included
"pipe/p_compiler.h", but src/gallium/include was missing from the C++
include path.

Problem 2: Add -std=c++11 to AM_CXXFLAGS. Android's libbacktrace headers
require C++11, but the Android toolchain (at least in the Chrome OS SDK)
does not enable C++11 by default.

v2: Add -std=c++11.

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Cc: Eric Engestrom <eric.engestrom@intel.com>