Create ANDROID build option

CMake seems not to support Android as a target and this option
lets us test this in CMakeLists.txt.
Use git__insertsort_r on Android too.
Define S_IREAD i S_IWRITE for Android.
Add instructions about buiding for Android to README.md
Merge pull request #1823 from kadamski/building4android

Small changes enabling compiling libgit2 for Android.