Define ANDROID_NDK CMake variable when building for Android

Versions of SDL since commit 5b65e0af01873e26673c11da43001e7075986290
fail to build if this isn't set. Let's take the value from the
ANDROID_NDK_HOME environment variable, which seems to be the standard.

Update bundled SDL2 to release 2.26.4

v0.35.2

Bump to v0.27

Updated version in README

Bumped version

Version 0.1.0 - Compatible with Rust beta

Rust version 0.11

Merge pull request Rust-SDL2#65 from drbawb/exp/fix-append-one

Use new `Vec#append_one()` method

Release 0.9