Disable libyaml GCC unused-value warning temporarily (#14579)

A Fix is proposed upstream and pending review.

ci_extra: build_test_all_windows
diff --git a/build_tools/third_party/libyaml/CMakeLists.txt b/build_tools/third_party/libyaml/CMakeLists.txt
index f296963..93eda90 100644
--- a/build_tools/third_party/libyaml/CMakeLists.txt
+++ b/build_tools/third_party/libyaml/CMakeLists.txt
@@ -41,4 +41,7 @@
 
 if(MSVC)
   target_compile_options(yaml_yaml PRIVATE /wd4996)
+elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+  # TODO(libyaml#210): Remove this option.
+  target_compile_options(yaml_yaml PRIVATE "-Wno-unused-value")
 endif()