Include 'string' in 'java.hh'

This fixes a build breakage on Darwin 13.0 (OS X 10.9 Mavericks).

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Use 'pkg-config' for 'libzip' include directories

'libzip' installs the platform specific header file 'zipconf.h' to
a non-standard location by default. Use 'pkg-config' to resolve the
'libzip' include directories.

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