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 3.3 Including Other Makefiles
 The `include' directive tells `make' to suspend reading the current
 makefile and read one or more other makefiles before continuing.  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 a tab is not allowed.  (If the line begins with a tab, it will be
 considered a command 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.   How to Use Variables
 Using Variables.
    For example, if you have three `.mk' files, `', `', and
 `', and `$(bar)' expands to `bish bash', then the following
      include foo *.mk $(bar)
    is equivalent to
      include foo bish bash
    When `make' processes an `include' directive, it suspends reading of
 the containing makefile and reads from each listed file in turn.  When
 that is finished, `make' resumes reading the makefile in which the
 directive appears.
    One occasion for using `include' directives is when several programs,
 handled by individual makefiles in various directories, need to use a
 common set of variable definitions ( Setting Variables Setting.)
 or pattern rules ( Defining and Redefining Pattern Rules Pattern
    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'.
  Automatic Prerequisites.  
    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 ( Summary of Options Options
 Summary.).  Then the following directories (if they exist) are
 searched, in this order: `PREFIX/include' (normally `/usr/local/include'
 (1)) `/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.   How Makefiles Are Remade
 Remaking Makefiles.  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
 and 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 do not exist.  For
 compatibility with some other `make' implementations, `sinclude' is
 another name for `-include'.
    ---------- Footnotes ----------
    (1) GNU Make compiled for MS-DOS and MS-Windows behaves as if PREFIX
 has been defined to be the root of the DJGPP tree hierarchy.
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