fix(boost): add compatibility for Boost.Process v1 API in Boost 1.89+

Boost 1.88 replaced Boost.Process with v2, breaking the v1 API.
Boost 1.89 restores v1 compatibility via <boost/process/v1.hpp>.

- Fail configuration if Boost 1.88 is detected
- Define FAIRMQ_BOOST_PROCESS_V1_HEADER for Boost >= 1.89
- Use conditional includes to select v1.hpp or process.hpp
- Add namespace aliases (bp, bp_this) for portable API access

test/device/_signals.cxx

@@ -9,7 +9,11 @@
 #include "runner.h"
 
 #include <gtest/gtest.h>
+#ifdef FAIRMQ_BOOST_PROCESS_V1_HEADER
+#include <boost/process/v1.hpp>
+#else
 #include <boost/process.hpp>
+#endif
 #include <fairmq/tools/Unique.h>
 #include <fairmq/tools/Process.h>