Examples can't be linked with libmbedtls but need libmbedcrypto, and
any users of libssh2 which use libtool and libssh2.la would encounter
the same problem.
This changes the mbedtls detection to search for libmbedcrypto, which
is the actual dependency for the backend.
The new --with-crypto option replaces the previous backend-specific
--with-{openssl,libgcrypt,mbedtls,wincng} options and fixes some issues.
* libgcrypt or mbedtls would previously be used whenever found, even
if configure was passed --without-libgcrypt or --without-mbedtls.
* If --with-$backend was specified then configure would not fail even
if that library could not be found, and would instead use whichever
crypto library was found first.
The new option defaults to `auto`, which makes configure check for all
supported crypto libraries in turn, choosing the first one found, or
exiting with an error if none can be found.
