3.3 Including Other Makefiles（包含其他的makefile文件）
The include directive tells make to suspend（挂起） reading the current makefile and read one or more other makefiles before continuing（include指令使make停止读取当前的makefile文件，而是接着读取被包含的makefile文件）. The directive is a line in the makefile that looks like this:
include filenames…filenames can contain shell file name patterns. If filenames is empty, nothing is included and no error is printed.
Extra spaces are allowed and ignored at the beginning of the line, but the first character must not be a tab (or the value of .RECIPEPREFIX)—if the line begins with a tab, it will be considered a recipe line. Whitespace is required between include and the file names, and between file names; extra whitespace is ignored there and at the end of the directive. A comment starting with ‘#’ is allowed at the end of the line. If the file names contain any variable or function references, they are expanded.
For example, if you have three .mk files, a.mk, b.mk, and c.mk, and $(bar) expands to bish bash, then the following expression
include foo *.mk $(bar)is equivalent to
include foo a.mk b.mk c.mk bish bashWhen make processes an include directive, it suspends reading of the containing makefile and reads from each listed file in turn（按顺序）. When that is finished, make resumes（继续） reading the makefile in which the directive appears.
One occasion（时机） for using include directives is when several programs, handled by individual makefiles in various directories, need to use a common set of variable definitions or pattern rules（使用include一种场景是，当需要同时编译好几个程序时，而这每个程序又都有自己的makefile，这时候可以使用include来包含各自的makefile文件）.
Another such occasion is when you want to generate prerequisites from source files automatically; the prerequisites can be put in a file that is included by the main makefile. This practice is generally cleaner than that of somehow appending the prerequisites to the end of the main makefile as has been traditionally done with other versions of make.
If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the ‘-I’ or ‘--include-dir’ option are searched. Then the following directories (if they exist) are searched, in this order: prefix/include (normally /usr/local/include) /usr/gnu/include, /usr/local/include, /usr/include.
If an included makefile cannot be found in any of these directories, a warning message is generated, but it is not an immediately fatal error; processing of the makefile containing the include continues. Once it has finished reading makefiles, make will try to remake any that are out of date or don’t exist. Only after it has tried to find a way to remake a makefile and failed, will make diagnose the missing makefile as a fatal error.
If you want make to simply ignore a makefile which does not exist or cannot be remade, with no error message, use the -include directive instead of include, like this:
-include filenames…This acts like include in every way except that there is no error (not even a warning) if any of the filenames (or any prerequisites of any of the filenames) do not exist or cannot be remade.
For compatibility with some other make implementations, sinclude is another name for -include.