This commit drops the previous custom CMake toolchain file for Android
in favor of CMake's new built-in toolchain for this (CMake >3.7.2).
This makes building SFML for Android a lot simpler and more straight
forward, working almost as smooth as other platforms.
To configure your build directory, all you have to do is defining just a
few variables the first time you invoke CMake.
**Required Variables**
* `CMAKE_SYSTEM_NAME` must be `Android`, so CMake knows we actually want
to cross-compile.
* `CMAKE_ANDROID_NDK` must point to the NDK's installation directory,
e.g. `/usr/android/ndk` or `c:/android/ndk`.
**Recommended Variables**
* `CMAKE_ANDROID_STL_TYPE` defines the STL implementation to be used.
You should use `c++_shared`, although others might work.
**Optional Variables**
* `CMAKE_SYSTEM_VERSION` can be set to pick a specific SDK version other
than the latest.
* `CMAKE_ANDROID_ARCH_ABI` defines the target architecture and ABI, for
example `armeabi` or `armeabi-v7a`.
Based on your system, you might want to enforce a specific generator to
prevent issues, e.g. using `MinGW Makefiles`.
* Updated the Android toolchain file to support NDKs up to the latest release (r12b; based on https://github.com/gongminmin/android-cmake).
* Fixed missing sRGB extension defines - also SFML once again compiles for older target API levels not having the sRGB extensions (fixes#1079, supersedes #1085).
* Changed SFML's default STL runtime to `stlport_shared`, since `c++_shared` is no longer supported.
