语句
####


条件语句
========

条件语句的语法::

    1. # if ...:
    <conditional-directive>
    <text-if-true>
    endif

    2. # if ... else ...:
    <conditional-directive>
    <text-if-true>
    else
    <text-if-false>
    endif

1. ifeq：条件判断，判断是否相等::

    示例: 判断当前编译器是否 gcc ，然后指定不同的库文件:
    ifeq ($(CC),gcc)
      libs=$(libs_for_gcc)
    else
      libs=$(normal_libs)
    endif

    例:
    判断变量 ROOT_PACKAGE 是否为空:
    ifeq ($(ROOT_PACKAGE),)


2. ifneq：条件判断，判断是否不相等::

    格式:
    ifneq (<arg1>, <arg2>)
    ifneq '<arg1>' '<arg2>'
    ifneq "<arg1>" "<arg2>"
    

3. ifdef：条件判断，判断变量是否已定义::

    格式:
    ifdef <variable-name>

4. ifndef：条件判断，判断变量是否未定义::

    格式:
    ifndef <variable-name>



循环语句
========

::

    LIST = one two three
    all:
        for i in $(LIST); do \
            echo $$i; \
        done

    # 等同于

    all:
        for i in one two three; do \
            echo $i; \
        done