Use libcrypto's checksum implementations if available, for speed.

Use libcrypto's checksum implementations if available, for speed.

On commonly used platform libcrypto uses architecture-specific
assembly code, which is significantly faster than the C code we
were using.  See Pádraig Brady's note in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-12/msg00000.html>.
Merge from gnulib, incorporating:
2013-12-07 md5, sha1, sha256, sha512: add gl_SET_CRYPTO_CHECK_DEFAULT
2013-12-07 md5, sha1, sha256, sha512: add 'auto', and set-default method
2013-12-04 include_next: minimize code duplication
2013-12-03 md5, sha1, sha256, sha512: support mandating use of openssl
2013-12-02 md5, sha1, sha256, sha512: use openssl routines if available
* configure.ac (--without-all): Set with_openssl_default too.
Use gl_SET_CRYPTO_CHECK_DEFAULT to default to 'auto'.
(HAVE_LIB_CRYPTO): New var.
Say whether Emacs is configured to use a crypto library.
* lib/gl_openssl.h, m4/absolute-header.m4, m4/gl-openssl.m4:
New files, copied from gnulib.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/md5.c, lib/md5.h, lib/sha1.c, lib/sha1.h:
* lib/sha256.c, lib/sha256.h, lib/sha512.c, lib/sha512.h:
* m4/include_next.m4, m4/md5.m4, m4/sha1.m4, m4/sha256.m4, m4/sha512.m4:
Update from gnulib.
* src/Makefile.in (LIB_CRYPTO): New macro.
(LIBES): Use it.