configure.ac: Avoid po-directories error
configure.ac: Avoid po-directories error

Avoid the po-directories error by putting AM_PO_SUBDIRS *before*
AM_GNU_GETTEXT:

configure.ac:NNN: error: `po-directories' is already registered with AC_CONFIG_COMMANDS.

gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2