build: Update tools, fixing configure and Makefiles.

* README_developer: Update.
* configure.ac (AH_BOTTOM): Add GTK_DISABLE_DEPRECATED define.
(AC_USE_SYSTEM_EXTENSIONS): Replace, instead of AC_GNU_SOURCE.
(GUILE_PKG, GUILE_FLAGS): Use this instead of PKG_CHECK_MODULES.
(AC_CONFIG_FILES): New.
(AC_OUTPUT): No args, with AC_CONFIG_FILES.
* icons/automake.mk (uninstall-icons): Add check with DESTDIR.
* po/automake.mk: Fix call of XGETTEXT with icons/gen-dot-desktop.scm.
Having --directory is enough.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>