Make strerror_r portable

strerror_r() can return either 'int' or 'char *' depending on its
implementation. The code sample was taken from AOSP.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Android build: include only jni.h

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Merge pull request #8 from yegorich/android-build

Android build