stop building s2n via aws_prebuild_dependency()

CMakeLists.txt

@@ -83,8 +83,8 @@ if(BUILD_DEPS)
     add_subdirectory(crt/aws-c-common)
 
     if(UNIX AND NOT APPLE AND NOT BYO_CRYPTO)
-        include(AwsPrebuildDependency)
         if(NOT USE_OPENSSL)
+            include(AwsPrebuildDependency)
 
             set(AWSLC_CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
 
@@ -116,14 +116,8 @@ if(BUILD_DEPS)
             )
         endif()
 
-        # prebuild s2n-tls.
-        aws_prebuild_dependency(
-            DEPENDENCY_NAME S2N
-            SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/crt/s2n
-            CMAKE_ARGUMENTS
-                -DUNSAFE_TREAT_WARNINGS_AS_ERRORS=OFF
-                -DBUILD_TESTING=OFF
-        )
+        set(UNSAFE_TREAT_WARNINGS_AS_ERRORS OFF CACHE BOOL "Disable warnings-as-errors when building S2N")
+        add_subdirectory(crt/s2n)
     endif()
 
     add_subdirectory(crt/aws-c-sdkutils)