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.