Fetching should still respect RSTUDIO_USE_SYSTEM_YAML_CPP=ON.

diff --git a/src/cpp/CMakeLists.txt b/src/cpp/CMakeLists.txt
index 5fa2649..f509421 100644
--- a/src/cpp/CMakeLists.txt
+++ b/src/cpp/CMakeLists.txt
@@ -197,7 +197,7 @@ endif()
 # yaml-cpp
 set(YAML_CPP_VERSION "0.8.0")
 set(YAML_CPP_REVISION "f7320141120f720aecc4c32be25586e7da9eb978")
-if(CMAKE_VERSION VERSION_GREATER "3.14")
+if(NOT RSTUDIO_USE_SYSTEM_YAML_CPP AND CMAKE_VERSION VERSION_GREATER "3.14")
 
    if(WIN32)
       add_definitions(-DYAML_CPP_STATIC_DEFINE)