JNI Build:
 - Update JNI build script to use GCC 4.6, required for
   NDK r9b and higher (ticket #1202)
 - Add more checks in build script, to fail quicker on problems
 - Move default NDK build location up one directory level
 - Fix setting JAVA_HOME
 - Fixes for running script in the directory

 I did not replace the checked-in 4.4.3 libjbigi.so with
 the new 4.6 version, as there's no need.
 The 4.6 version will be tested by nextloop in the F-Droid build.