Default libcrypt to whether or not we find it

Also deliberately don't provide the function at all if we aren't
compiled with libcrypt so that we can't link if we try to call it when
not available.

external/CMakeLists.txt

@@ -139,7 +139,10 @@ endif()
 set(default_libcrypt OFF)
 
 if(LINUX AND NOT STATIC_LINK)
-  set(default_libcrypt ON)
+  pkg_check_modules(LIBCRYPT libcrypt IMPORTED_TARGET)
+  if(LIBCRYPTO_FOUND)
+      set(default_libcrypt ON)
+  endif()
 endif()
 if(MACOS)
   set(default_libcrypt ON)

llarp/crypto/crypto.cpp

@@ -499,13 +499,11 @@ namespace llarp
     crypto_kem_keypair(d + PQ_SECRETKEYSIZE, d);
   }
 
+#ifdef HAVE_CRYPT
   bool
   crypto::check_passwd_hash(std::string pwhash, std::string challenge)
   {
-    (void)pwhash;
-    (void)challenge;
     bool ret = false;
-#ifdef HAVE_CRYPT
     auto pos = pwhash.find_last_of('$');
     auto settings = pwhash.substr(0, pos);
     crypt_data data{};
@@ -514,9 +512,9 @@ namespace llarp
       ret = ptr == pwhash;
     }
     sodium_memzero(&data, sizeof(data));
-#endif
     return ret;
   }
+#endif
 
   const byte_t*
   seckey_topublic(const SecretKey& sec)

llarp/service/auth.cpp

@@ -125,7 +125,9 @@ namespace llarp::service
         case AuthFileType::eAuthFilePlain:
           return hash == challenge;
         case AuthFileType::eAuthFileHashes:
+#ifdef HAVE_CRYPT
           return crypto::check_passwd_hash(std::move(hash), std::move(challenge));
+#endif
         default:
           return false;
       }