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

src/crypto.h

@@ -40,22 +40,16 @@
 
 #if defined(LIBSSH2_OPENSSL) || defined(LIBSSH2_WOLFSSL)
 #include "openssl.h"
-#endif
-
-#ifdef LIBSSH2_LIBGCRYPT
+#elif defined(LIBSSH2_LIBGCRYPT)
 #include "libgcrypt.h"
-#endif
-
-#ifdef LIBSSH2_WINCNG
-#include "wincng.h"
-#endif
-
-#ifdef LIBSSH2_OS400QC3
-#include "os400qc3.h"
-#endif
-
-#ifdef LIBSSH2_MBEDTLS
+#elif defined(LIBSSH2_MBEDTLS)
 #include "mbedtls.h"
+#elif defined(LIBSSH2_OS400QC3)
+#include "os400qc3.h"
+#elif defined(LIBSSH2_WINCNG)
+#include "wincng.h"
+#else
+#error "no cryptography backend selected"
 #endif
 
 #ifdef LIBSSH2_NO_MD5