diff --git a/crt/CMakeLists.txt b/crt/CMakeLists.txt
index 6dc50883..875068d3 100644
--- a/crt/CMakeLists.txt
+++ b/crt/CMakeLists.txt
@@ -30,9 +30,9 @@ include(CTest)
 # (On Windows and Apple we use the default OS libraries)
 if(UNIX AND NOT APPLE)
     option(USE_OPENSSL "Set this if you want to use your system's OpenSSL compatible libcrypto" OFF)
+    include(AwsPrebuildDependency)
 
     if(NOT USE_OPENSSL)
-        include(AwsPrebuildDependency)
 
         set(AWSLC_CMAKE_ARGUMENTS
             -DDISABLE_GO=ON  # Build without using Go, we don't want the extra dependency
@@ -54,8 +54,14 @@ if(UNIX AND NOT APPLE)
         )
     endif()
 
-    set(UNSAFE_TREAT_WARNINGS_AS_ERRORS OFF CACHE BOOL "")
-    add_subdirectory(s2n)
+    # prebuild s2n-tls.
+    aws_prebuild_dependency(
+        DEPENDENCY_NAME S2N
+        SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/s2n
+        CMAKE_ARGUMENTS
+            -DUNSAFE_TREAT_WARNINGS_AS_ERRORS=OFF
+            -DBUILD_TESTING=OFF
+    )
 endif()
 
 add_subdirectory(aws-c-common)