Fixed variable name type (ax_prog_javah_bin_dir) and follow javah symlin...

Fixed variable name type ('ax_prog_javah_bin_dir' was 'ax_prog_javah_bin') and follow javah symlink to java installation folder to find jni.h.

On my system javah is actually /usr/bin/javah, which is a symlink.  So it wouldn't find javah/../../include.