support --without-openssl at configure time

Disables and removes dependency on OpenSSL. Many features don't
work and the set of crypto options is greatly restricted. This
will only work on system with native arc4random or /dev/urandom.

Considered highly experimental for now.
diff --git a/Makefile.in b/Makefile.in
index ebd48c3..d7ba813 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -76,7 +76,7 @@
 
 LIBSSH_OBJS=${LIBOPENSSH_OBJS} \
 	authfd.o authfile.o bufaux.o bufbn.o buffer.o \
-	canohost.o channels.o cipher.o cipher-aes.o \
+	canohost.o channels.o cipher.o cipher-aes.o cipher-aesctr.o \
 	cipher-bf1.o cipher-ctr.o cipher-3des1.o cleanup.o \
 	compat.o compress.o crc32.o deattack.o fatal.o hostfile.o \
 	log.o match.o md-sha256.o moduli.o nchan.o packet.o \
@@ -87,7 +87,7 @@
 	msg.o progressmeter.o dns.o entropy.o gss-genr.o umac.o umac128.o \
 	ssh-pkcs11.o smult_curve25519_ref.o \
 	kexc25519.o kexc25519c.o poly1305.o chacha.o cipher-chachapoly.o \
-	ssh-ed25519.o digest-openssl.o hmac.o \
+	ssh-ed25519.o digest-openssl.o digest-libc.o hmac.o \
 	sc25519.o ge25519.o fe25519.o ed25519.o verify.o hash.o blocks.o
 
 SSHOBJS= ssh.o readconf.o clientloop.o sshtty.o \