better handling of includes and system include directories

CMake/cotire.cmake

@@ -836,21 +836,29 @@ macro (cotire_add_definitions_to_cmd _cmdVar _language)
 	endforeach()
 endmacro()
 
-macro (cotire_add_includes_to_cmd _cmdVar _language _includesVar _systemIncludesVar)
-	foreach (_include ${${_includesVar}})
-		if (WIN32 AND CMAKE_${_language}_COMPILER_ID MATCHES "MSVC|Intel")
-			file (TO_NATIVE_PATH "${_include}" _include)
-			list (APPEND ${_cmdVar} "${CMAKE_INCLUDE_FLAG_${_language}}${CMAKE_INCLUDE_FLAG_${_language}_SEP}${_include}")
-		else()
-			list (FIND ${_systemIncludesVar} "${_include}" _index)
-			if (_index GREATER -1 AND CMAKE_INCLUDE_SYSTEM_FLAG_${_language})
-				list (APPEND ${_cmdVar} "${CMAKE_INCLUDE_SYSTEM_FLAG_${_language}}${_include}")
-			else()
+function (cotire_add_includes_to_cmd _cmdVar _language _includesVar _systemIncludesVar)
+	set (_includeDirs ${${_includesVar}} ${${_systemIncludesVar}})
+	if (_includeDirs)
+		list (REMOVE_DUPLICATES _includeDirs)
+		foreach (_include ${_includeDirs})
+			if (WIN32 AND CMAKE_${_language}_COMPILER_ID MATCHES "MSVC|Intel")
+				file (TO_NATIVE_PATH "${_include}" _include)
 				list (APPEND ${_cmdVar} "${CMAKE_INCLUDE_FLAG_${_language}}${CMAKE_INCLUDE_FLAG_${_language}_SEP}${_include}")
+			else()
+				set (_index -1)
+				if ("${CMAKE_INCLUDE_SYSTEM_FLAG_${_language}}" MATCHES ".+")
+					list (FIND ${_systemIncludesVar} "${_include}" _index)
+				endif()
+				if (_index GREATER -1)
+					list (APPEND ${_cmdVar} "${CMAKE_INCLUDE_SYSTEM_FLAG_${_language}}${_include}")
+				else()
+					list (APPEND ${_cmdVar} "${CMAKE_INCLUDE_FLAG_${_language}}${CMAKE_INCLUDE_FLAG_${_language}_SEP}${_include}")
+				endif()
 			endif()
-		endif()
-	endforeach()
-endmacro()
+		endforeach()
+	endif()
+	set (${_cmdVar} ${${_cmdVar}} PARENT_SCOPE)
+endfunction()
 
 macro (cotire_add_frameworks_to_cmd _cmdVar _language)
 	if (APPLE)