fix darwin boost break on std::piecewise_construct

Summary:
Since Xcode 15.3 std::piecewise_construct is only visible in c++17 and later modes

std::piecewise_construct used to be visible in c++03 and later as extensions, see
https://developer.apple.com/documentation/xcode-release-notes/xcode-15_3-release-notes#Deprecations

Reviewed By: lebentle

Differential Revision: D56539492

fbshipit-source-id: 25706bec5aa5a4343a8429eb3d3eeeae9d9a0469

build/fbcode_builder/manifests/boost

@@ -105,6 +105,8 @@ pch=off
 
 [b2.args.os=darwin]
 toolset=clang
+# Since Xcode 15.3 std::piecewise_construct is only visible in C++17 and later modes
+cxxflags="-DBOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT=0"
 
 [b2.args.all(os=windows,fb=on)]
 toolset=msvc-14.2