build: be friendly with 3rd-party build tools

After recent build changes, 3rd party build that took the list of
C source to compile them as-is, stopped working as expected, due to
`blowfish.c` and crypto-backend C sources no longer expected to compile
separately but via `bcrypt_pbkdf.c` and `crypto.c`, respectively.

This patch ensures that compiling these files directly result in an
empty object instead of redundant code and duplicated symbols.

Also:
- add a compile-time error if none of the supported crypto backends
  are enabled.
- fix `libssh2_crypto_engine()` for wolfSSL and os400qc3.
  Rearrange code to avoid a hard-to-find copy of crypto-backend
  selection guards.

Follow-up to 4f0f4bff5a
Follow-up to ff3c774e03

Closes #951

include/libssh2.h

@@ -1010,7 +1010,8 @@ typedef enum {
     libssh2_openssl,
     libssh2_gcrypt,
     libssh2_mbedtls,
-    libssh2_wincng
+    libssh2_wincng,
+    libssh2_os400qc3
 } libssh2_crypto_engine_t;
 
 LIBSSH2_API