- Add sysroot path to environment variable 'CPPFLAGS':
When checking C preprocessor, the 'configure' script of libbacktrace uses 'CPPFLAGS' rather than 'CFLAGS' and doesn't get the correct search path. (#15758)
This check passes on Linux because the '/lib/cpp' fallback is available there.
- Introduce CMake toolchain file for Android cross compiling:
CMake needs several variables [1] to cross compile for Android.
It works (accidentally) on Linux because cmake-rs sets compilers correctly and binutils for Linux & Android are pretty much the same.
[1] https://cmake.org/cmake/help/v3.7/manual/cmake-toolchains.7.html#cross-compiling-for-android-with-the-ndk
