THRIFT-3097 cmake targets unconditionally depend on optional libraries
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2800167..f19f999 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -75,7 +75,11 @@
 if(WITH_CPP)
     add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/lib/cpp)
     if(BUILD_TESTING)
-        add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/test/cpp)
+        if(WITH_LIBEVENT)
+            add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/test/cpp)
+        else()
+            message(WARNING "libevent not found or disabled; will not build some tests")
+        endif()
     endif()
 endif()