Add support for libc++, the LLVM C++ Standard Library.

[matt-langston]

This PR adds support for the LLVM C++ Standard Library reported in #17. My company is using this successfully with the CrystaX NDK (required for boost) for our Android builds. Use it like this:

cmake -DCMAKE_TOOLCHAIN_FILE=android.toolchain.cmake \
-DANDROID_NDK=<ndk_path> \
-DCMAKE_BUILD_TYPE=Release \
-DANDROID_ABI="armeabi-v7a" \
-DANDROID_TOOLCHAIN_NAME=arm-linux-androideabi-clang3.5 \
-DANDROID_STL=libc++_shared \
<source_path>
cmake --build .

You can also replace -DANDROID_STL=libc++_shared with -DANDROID_STL=libc++_static.

I suggest that this PR supersede PR #33.

[abergmeier]

I think it is confusing to use other options, than the Application.mk does.

[taka-no-me]

This is quite incomplete libc++ support. And as pointed by @abergmeier the naming should be consistent with NDK. So I'm closing this one.