tai/taia: minor cleanup
configure: add ACX_HEADER_ENDIAN macro

 * Add and use ACX_HEADER_ENDIAN macro to find the appropriate
   endian.h.
 * Change tai/taia packing methods to use endian.h methods.
cleanup headers

time-private.h is for building the library, while time.h is for
using the library.
tai/taia: rename struct seconds property
interface: java: remove pkgpath make variable

Remove the pkgpath make variable and replace all usage of it with
sv/time.  After enabling subdir-objects, the generated Makefile
was looking for dependencies in $(pkgpath) which wasn't evaluated
during configure time and was causing the build to fail.