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(autoconf.info)Build Directories

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Build Directories
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   You can support compiling a software package for several
architectures simultaneously from the same copy of the source code.
The object files for each architecture are kept in their own directory.

   To support doing this, `make' uses the `VPATH' variable to find the
files that are in the source directory.  GNU `make' and most other
recent `make' programs can do this.  Older `make' programs do not
support `VPATH'; when using them, the source code must be in the same
directory as the object files.

   To support `VPATH', each `Makefile.in' should contain two lines that
look like:

     srcdir = @srcdir@
     VPATH = @srcdir@

   Do not set `VPATH' to the value of another variable, for example
`VPATH = $(srcdir)', because some versions of `make' do not do variable
substitutions on the value of `VPATH'.

   `configure' substitutes in the correct value for `srcdir' when it
produces `Makefile'.

   Do not use the `make' variable `$<', which expands to the pathname
of the file in the source directory (found with `VPATH'), except in
implicit rules.  (An implicit rule is one such as `.c.o', which tells
how to create a `.o' file from a `.c' file.)  Some versions of `make'
do not set `$<' in explicit rules; they expand it to an empty value.

   Instead, `Makefile' command lines should always refer to source
files by prefixing them with `$(srcdir)/'.  For example:

     time.info: time.texinfo
             $(MAKEINFO) $(srcdir)/time.texinfo