Add a GNOME3 pinentry based on gcr.
* configure.ac (--enable-pinentry-gnome3): Option to enable the GNOME3 pinentry. (pinentry_gnome_3): Set to yes if enabled and gcr-3 and gcr-base-3 gcr is available. (GNOME3CFLAGS): Define and AC_SUBST. (GNOME3LIBS): Define and AC_SUBST. (GCR_API_SUBJECT_TO_CHANGE): Define. (BUILD_PINENTRY_GNOME_3): Define. * Makefile.am (pinentry_gnome_3): Define. (SUBDIRS): Add ${pinentry_gnome_3}. * gnome3/Makefile.am: New file. * gnome3/pinentry-gnome3.c: New file.