4.3 Types of Prerequisites
==========================
There are actually two different types of prerequisites understood by GNU `make': normal prerequisites such as described in the previous section, and "order-only" prerequisites. A normal prerequisite makes two statements: first, it imposes an order in which recipes will be invoked: the recipes for all prerequisites of a target will be completed before the recipe for the target is run. Second, it imposes a dependency relationship: if any prerequisite is newer than the target, then the target is considered out-of-date and must be rebuilt.
Normally, this is exactly what you want: if a target's prerequisite
is updated, then the target should also be updated.
==========================
GNU make 可以理解两种不同类型的前提条件:
前面章节中描述的普通的前提条件，以及 仅排序 前提条件。
一个普通的前提条件完成两件事：首先，它确立了一个顺序，片段被激活的时候要运用此顺序:
一个目的的所有前提条件都将被完成后，此目的的片段才会执行。
第二，它确立了一个依赖关系: 如果任何前提条件比目的新，则目的被认为已经过期，需要重新构建。
通常，这正是你所想要的：如果一个前提条件被修改了，那么目的也需要被修改。
本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2012/09/18/2690756.html，如需转载请自行联系原作者