(make.info.gz) Overriding Makefiles

 
 3.8 Overriding Part of Another Makefile
 =======================================
 
 Sometimes it is useful to have a makefile that is mostly just like
 another makefile.  You can often use the `include' directive to include
 one in the other, and add more targets or variable definitions.
 However, if the two makefiles give different commands for the same
 target, `make' will not let you just do this.  But there is another way.
 
    In the containing makefile (the one that wants to include the other),
 you can use a match-anything pattern rule to say that to remake any
 target that cannot be made from the information in the containing
 makefile, `make' should look in another makefile.   Pattern
 Rules, for more information on pattern rules.
 
    For example, if you have a makefile called `Makefile' that says how
 to make the target `foo' (and other targets), you can write a makefile
 called `GNUmakefile' that contains:
 
      foo:
              frobnicate > foo
 
      %: force
              @$(MAKE) -f Makefile $@
      force: ;
 
    If you say `make foo', `make' will find `GNUmakefile', read it, and
 see that to make `foo', it needs to run the command `frobnicate > foo'.
 If you say `make bar', `make' will find no way to make `bar' in
 `GNUmakefile', so it will use the commands from the pattern rule: `make
 -f Makefile bar'.  If `Makefile' provides a rule for updating `bar',
 `make' will apply the rule.  And likewise for any other target that
 `GNUmakefile' does not say how to make.
 
    The way this works is that the pattern rule has a pattern of just
 `%', so it matches any target whatever.  The rule specifies a
 prerequisite `force', to guarantee that the commands will be run even
 if the target file already exists.  We give `force' target empty
 commands to prevent `make' from searching for an implicit rule to build
 it--otherwise it would apply the same match-anything rule to `force'
 itself and create a prerequisite loop!
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