Replace target_var boolean with enum variable_scope

Replace target_var boolean with enum variable_scope

Replace the target_var boolean with an enum to distinguish between
global, target-specific, and pattern-specific variables when defining.

* src/variable.h (enum variable_scope): Introduce enum variable_scope.
Replace parameter target_var of type int with enum variable_scope.
* src/load.c (load_file): Ditto.
* src/main.c (handle_non_switch_argument): Ditto.
* src/read.c (eval_makefile): Ditto.
(eval): Ditto.
(do_define): Ditto.
(record_target_var): Ditto.
(construct_include_path): Ditto.
* src/variable.c (initialize_file_variables): Ditto.
(shell_result): Ditto.
(try_variable_definition): Ditto.
(do_variable_definition): Ditto.