From: stuff overlay maintainer
Subject: Build against Boost.Process 1.86+ default v2

Boost 1.86 made boost::process default to v2, which is a different API.
OrcaSlicer still uses the v1 API (child, ipstream, pipe, search_path,
std_in/out/err redirection operators). Define BOOST_PROCESS_VERSION=1
to make the v1 namespace inline (so boost::process::child resolves to
boost::process::v1::child) and switch the umbrella header to
<boost/process/v1.hpp>, since <boost/process.hpp> in 1.86+ only pulls
in the v2 surface.

--- a/src/libslic3r/GCode/PostProcessor.cpp
+++ b/src/libslic3r/GCode/PostProcessor.cpp
@@ -149,7 +149,8 @@
 
 #include <cstdlib>   // getenv()
 #include <sstream>
-#include <boost/process.hpp>
+#define BOOST_PROCESS_VERSION 1
+#include <boost/process/v1.hpp>
 
 namespace process = boost::process;
 
--- a/src/slic3r/GUI/MediaPlayCtrl.cpp
+++ b/src/slic3r/GUI/MediaPlayCtrl.cpp
@@ -15,7 +15,8 @@
 #include <boost/nowide/cstdio.hpp>
 #include <boost/nowide/utf8_codecvt.hpp>
 #undef pid_t
-#include <boost/process.hpp>
+#define BOOST_PROCESS_VERSION 1
+#include <boost/process/v1.hpp>
 #ifdef __WIN32__
 #include <boost/process/windows.hpp>
 #else
--- a/src/slic3r/GUI/RemovableDriveManager.cpp
+++ b/src/slic3r/GUI/RemovableDriveManager.cpp
@@ -22,7 +22,8 @@
 #include <pwd.h>
 #include <boost/filesystem.hpp>
 #include <boost/system/error_code.hpp>
-#include <boost/process.hpp>
+#define BOOST_PROCESS_VERSION 1
+#include <boost/process/v1.hpp>
 #endif
 
 namespace Slic3r {