Use platform specific include directory

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Include 'string' in 'java.hh'

This fixes build breakage on Clang 5.0 on OS X 10.9 (Mavericks).

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>