build: New target "release" to automate the release process.

* Makefile.am (RELEASE_ARCHIVE_DIR): New.
(RELEASE_SIGNING_KEY): New.
(AM_DISTCHECK_CONFIGURE_FLAGS): Remove removed --enable-gpg2-is-gpg,
(RELEASE_NAME, RELEASE_W32_STEM_NAME): New.
(release, sign-release): New.

This requires GNU make and also some other decent utilities; however,
they are anyway required for building the W32 installer.

• Signed-off-by: Werner Koch <wk@gnupg.org>