Use build.sh to recursively build all samples.

The build script depends on the following environment variables to be set:
ANDROID_HOME to the path where the Android SDK is installed
NDK_ROOT to the path where the Android NDK is installed
NDK_MODULE_PATH to the location of the Play Games cpp libraries.

Note: NDK_MODULE_PATH should point one directory *above* where
you've installed gpg-cpp-sdk. This variable is used by ndk-build to locate
all installed modules, not just the Google Play Games library.