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(make.info)Wildcard Pitfall

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Pitfalls of Using Wildcards
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   Now here is an example of a naive way of using wildcard expansion,
that does not do what you would intend.  Suppose you would like to say
that the executable file `foo' is made from all the object files in the
directory, and you write this:

     objects = *.o
     
     foo : $(objects)
             cc -o foo $(CFLAGS) $(objects)

The value of `objects' is the actual string `*.o'.  Wildcard expansion
happens in the rule for `foo', so that each _existing_ `.o' file
becomes a prerequisite of `foo' and will be recompiled if necessary.

   But what if you delete all the `.o' files?  When a wildcard matches
no files, it is left as it is, so then `foo' will depend on the
oddly-named file `*.o'.  Since no such file is likely to exist, `make'
will give you an error saying it cannot figure out how to make `*.o'.
This is not what you want!

   Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
`wildcard' function and string substitution.  Note: The Function
`wildcard'.

   Microsoft operating systems (MS-DOS and MS-Windows) use backslashes
to separate directories in pathnames, like so:

       c:\foo\bar\baz.c

   This is equivalent to the Unix-style `c:/foo/bar/baz.c' (the `c:'
part is the so-called drive letter).  When `make' runs on these
systems, it supports backslashes as well as the Unix-style forward
slashes in pathnames.  However, this support does _not_ include the
wildcard expansion, where backslash is a quote character.  Therefore,
you _must_ use Unix-style slashes in these cases.