parse cmake environment variables version check

CMakeBBLM.cmake

@@ -160,22 +160,25 @@ foreach (_line IN LISTS _output)
 endforeach()
 
 # cmake environment variables
-execute_process(
-	COMMAND ${CMAKE_COMMAND} --help-manual cmake-env-variables
-	OUTPUT_VARIABLE _output
-	OUTPUT_STRIP_TRAILING_WHITESPACE)
-string (REPLACE "\n" ";" _output "${_output}")
-set (_variable "")
-foreach (_line IN LISTS _output)
-	if(_line MATCHES "^-+ *$")
-		if (_variable)
-			list (APPEND _variableList "${_variable}")
-			set (_variable "")
+if (NOT CMAKE_VERSION VERSION_LESS "3.0.0")
+	execute_process(
+		COMMAND ${CMAKE_COMMAND} --help-manual cmake-env-variables
+		OUTPUT_VARIABLE _output
+		OUTPUT_STRIP_TRAILING_WHITESPACE)
+	string (REPLACE "\n" ";" _output "${_output}")
+	set (_variable "")
+	foreach (_line IN LISTS _output)
+		if(_line MATCHES "^-+ *$")
+			if (_variable)
+				list (APPEND _variableList "${_variable}")
+				set (_variable "")
+			endif()
+		elseif (_line MATCHES "^([A-Za-z0-9_<>-]+) *$")
+			set (_variable "${_line}")
 		endif()
-	elseif (_line MATCHES "^([A-Za-z0-9_<>-]+) *$")
-		set (_variable "${_line}")
-	endif()
-endforeach()
+	endforeach()
+endif()
+
 # explicitly add variables missed by command parser
 list(APPEND _variableList "CMAKE_ARGV(0..9)" "CMAKE_MATCH_(0..9)")
 # add undocumented, but useful CMake variables