Minimal diff from upstream boost.m4 (https://github.com/tsuna/boost.m4)
to make things work with >=boost-1.89.0, which made boost_system a
header-only library.

--- a/m4/boost.m4~
+++ b/m4/boost.m4
@@ -1130,15 +1130,20 @@ BOOST_DEFUN([String_Algo],
 ])
 
 
-# BOOST_SYSTEM([PREFERRED-RT-OPT])
+# BOOST_SYSTEM([PREFERRED-RT-OPT], [ERROR_ON_UNUSABLE])
 # --------------------------------
 # Look for Boost.System.  For the documentation of PREFERRED-RT-OPT, see the
 # documentation of BOOST_FIND_LIB above.  This library was introduced in Boost
-# 1.35.0.
+# 1.35.0 and is header only since 1.70.
 BOOST_DEFUN([System],
-[BOOST_FIND_LIB([system], [$1],
+[
+if test $boost_major_version -ge 170; then
+  BOOST_FIND_HEADER([boost/system/error_code.hpp])
+else
+  BOOST_FIND_LIB([system], [$1],
                 [boost/system/error_code.hpp],
-                [boost::system::error_code e; e.clear();])
+                [boost::system::error_code e; e.clear();], [], [], [$2])
+fi
 ])# BOOST_SYSTEM