cmake: omit Boost::system on Boost 1.89+ where the compiled stub is gone.

Keep linking Boost::log, and only require Boost::system on older package sets that still ship libboost_system.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmake/BoostSharedDeps.cmake

@@ -0,0 +1,59 @@
+# EXPLANATION:
+# Shared Boost deps for standalone or nested libspinscale builds.
+# Always require Boost.Log as a shared library. Require Boost.System only on
+# versions that still ship a compiled stub (Ubuntu dropped it at Boost 1.89).
+#
+# Sets BOOST_SHARED_DEP_TARGETS for target_link_libraries(... ${BOOST_SHARED_DEP_TARGETS}).
+# Uses Boost:: imported targets (not a project INTERFACE lib) so export sets stay valid.
+
+set(BOOST_COMPILED_SYSTEM_REMOVED_VERSION "1.89.0")
+set(BOOST_SHARED_DEPS_MIN_VERSION "1.69")
+
+function(boostSharedDepsComponentsForVersion _boostVersion _outVar)
+	set(_components log)
+	if(_boostVersion VERSION_LESS "${BOOST_COMPILED_SYSTEM_REMOVED_VERSION}")
+		list(APPEND _components system)
+	endif()
+	set(${_outVar} ${_components} PARENT_SCOPE)
+endfunction()
+
+function(boostSharedDepsLinkTargets _outVar)
+	set(_targets Boost::log)
+	if(TARGET Boost::system)
+		list(APPEND _targets Boost::system)
+	endif()
+	set(${_outVar} ${_targets} PARENT_SCOPE)
+endfunction()
+
+function(boostSharedDepsReportStatus)
+	if(TARGET Boost::system)
+		message(STATUS
+			"Boost ${Boost_VERSION}: linking Boost::system "
+			"(compiled stub still present)")
+	else()
+		message(STATUS
+			"Boost ${Boost_VERSION}: omitting Boost::system "
+			"(header-only; compiled stub removed at "
+			"${BOOST_COMPILED_SYSTEM_REMOVED_VERSION})")
+	endif()
+endfunction()
+
+if(NOT BOOST_SHARED_DEPS_RESOLVED)
+	# Prefer shared Boost libs where a compiled component still exists.
+	set(Boost_USE_STATIC_LIBS OFF)
+	set(Boost_USE_HEADER_ONLY OFF)
+
+	# Resolve version before requesting components so we can skip system on
+	# Boost 1.89+, where Ubuntu no longer packages libboost_system.
+	find_package(Boost ${BOOST_SHARED_DEPS_MIN_VERSION} REQUIRED)
+	boostSharedDepsComponentsForVersion("${Boost_VERSION}" _boostSharedDepsComponents)
+	find_package(Boost ${BOOST_SHARED_DEPS_MIN_VERSION} REQUIRED
+		COMPONENTS ${_boostSharedDepsComponents})
+
+	boostSharedDepsLinkTargets(BOOST_SHARED_DEP_TARGETS)
+	set(BOOST_SHARED_DEPS_RESOLVED TRUE)
+	boostSharedDepsReportStatus()
+
+	# Ensure remaining Boost libs (e.g. Log) use dynamic linking.
+	add_compile_definitions(BOOST_ALL_DYN_LINK)
+endif()