apply user provided prefix header to unity build target

CMake/cotire.cmake

@@ -2731,6 +2731,20 @@ function (cotire_collect_unity_target_sources _target _languages _unityTargetSou
 	set (${_unityTargetSourcesVar} ${_unityTargetSources} PARENT_SCOPE)
 endfunction()
 
+function (cotire_setup_unity_target_pch_usage _languages _target)
+	foreach (_language ${_languages})
+		get_property(_unityFiles TARGET ${_target} PROPERTY COTIRE_${_language}_UNITY_SOURCE)
+		if (_unityFiles)
+			get_property(_userPrefixFile TARGET ${_target} PROPERTY COTIRE_${_language}_PREFIX_HEADER_INIT)
+			get_property(_prefixFile TARGET ${_target} PROPERTY COTIRE_${_language}_PREFIX_HEADER)
+			if (_userPrefixFile AND _prefixFile)
+				# user provided prefix header must be included unconditionally by unity sources
+				cotire_setup_prefix_file_inclusion(${_language} ${_target} "${_prefixFile}" ${_unityFiles})
+			endif()
+		endif()
+	endforeach()
+endfunction()
+
 function (cotire_setup_unity_build_target _languages _configurations _target)
 	get_target_property(_unityTargetName ${_target} COTIRE_UNITY_TARGET_NAME)
 	if (NOT _unityTargetName)
@@ -2956,6 +2970,9 @@ function (cotire_target _target)
 	if (_targetUsePCH)
 		cotire_setup_target_pch_usage("${_targetLanguages}" ${_target} ${_wholeTarget} ${_cmds})
 		cotire_setup_pch_target("${_targetLanguages}" "${_option_CONFIGURATIONS}" ${_target})
+		if (_targetAddSCU)
+			cotire_setup_unity_target_pch_usage("${_targetLanguages}" ${_target})
+		endif()
 	endif()
 	get_target_property(_targetAddCleanTarget ${_target} COTIRE_ADD_CLEAN)
 	if (_targetAddCleanTarget)