* 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.
* Replaced the toolchain file with a new version based on [zuhowei's fork](https://github.com/zhuowei/android-cmake), which enables x64 builds as well as support for the latest NDK. This breaks compatibility with old build directories.
* Removed the STL dependency from **sfml-activity** rather than relying on *some* implementation implicitly linked by default.
* Deleted *project.properties*, which wasn't supposed to be part of the repository code. You have to use the Android SDK to recreate it (`android update project --path to/your/example --target 1 --name SFML-Example`).
* Made it possible to select a STL implementation to be used (default: `c++_shared`). Keep in mind that not all available configurations are necessarily compatible with SFML.
* Fixed linker flags to be compatible with Nvidia's Nsight Tegra for Visual Studio.
* It is now possible to compile the Android version using Nvidia's Nsight Tegra for Visual Studio (requires up-to-date CMake and `CMAKE_SFML_SYSTEM` to be set to `Android`; keep in mind that this is still experimental and requires further CMake updates).
* Updated and renamed some Android specific CMake variables.
* Made `armeabi-v7a` the default ABI for Android builds.
- Cmake errored out due to the ANDROID_NDK_* variables being used before being set/detected for the first time.
- Fixed one warning regarding one string replace in CMake.
- Fixed warnings when compiling SFML-Activity.
Warning: I kept the same variable name because this file will be rewritten fairly soon since most of its code no longer applies to SFML and becomes incorrect (too hard to maintain).
Thus stlport variable names refers to libc++ stuff. E.g: ANDROID_USE_STLPORT=1 turns the use of libc++ on.
