md5, sha1, sha256, sha512: add gl_SET_CRYPTO_CHECK_DEFAULT

This provides a new way to specify the default for
gl_CRYPTO_CHECK, one that is reflected in the --help message.
Emacs uses this, as well as the old way.
This attempts to implement a suggestion by Pádraig Brady in
<http://lists.gnu.org/archive/html/coreutils/2013-12/msg00080.html>.
* m4/gl-openssl.m4(gl_SET_CRYPTO_CHECK_DEFAULT): New macro.
(gl_CRYPTO_CHECK): Use it.  Mention the default in --help output.