http://stackoverflow.com/questions/448910/makefile-variable-assignment
更加全面的介绍 http://blog.csdn.net/ruglcc/article/details/7814546/
这个是原作者http://blog.csdn.net/haoel/article/details/2896
Lazy Set
VARIABLE = value
Normal setting of a variable - values within it are recursively expanded when the variable is used, not when it's declared
Immediate Set
VARIABLE := value
Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.
Set If Absent
VARIABLE ?= value
Setting of a variable only if it doesn't have a value
Append
VARIABLE += value
Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)
还有一个在“嵌套执行”中比较有用的参数，“-w”或是“--print-directory”会在make的过程中输出一些信息，让你看到目前的工作目录。比如，如果我们的下级make目录是“/home/hchen/gnu/make”，如果我们使用“make -w”来执行，那么当进入该目录时，我们会看到：
make: Entering directory `/home/hchen/gnu/make'.
而在完成下层make后离开目录时，我们会看到：
make: Leaving directory `/home/hchen/gnu/make'
当你使用“-C”参数来指定make下层Makefile时，“-w”会被自动打开的。如果参数中有“-s”（“--slient”）或是“--no-print-directory”，那么，“-w”总是失效的。