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-$backend was specified then configure would not fail even
if that library could not be found, and would instead use whichever
crypto library was found first.
The new option defaults to `auto`, which makes configure check for all
supported crypto libraries in turn, choosing the first one found, or
exiting with an error if none can be found.
Commit d512b25f69a1b6778881f6b4b5ff9cfc6023be42 added automake
conditionals to Makefile.inc but since Makefile.inc is included
from Makefile for all other build systems that does not work.
This commit instead adds Makefile.OpenSSL.inc and Makefile.libgcrypt.inc
and moves the automake conditional to its proper place, src/Makefile.am.
The automake conditional includes the correct Makefile.$name.inc per
the crypto library selection/detection done by configure.
All non-autotools build system files in libssh2 are hardcoded to use
OpenSSL and do not get a conditional but at least there is some reuse
because they can all include the new Makefile.OpenSSL.inc.
