(autoconf.info)Setting Output Variables

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6.2 Setting Output Variables
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One way to record the results of tests is to set "output variables",
which are shell variables whose values are substituted into files that
'configure' outputs.  The two macros below create new output variables.
Note: Preset Output Variables, for a list of output variables that are
always available.

 -- Macro: AC_SUBST (VARIABLE)
     Create an output variable from a shell variable.  Make 'AC_OUTPUT'
     substitute the variable VARIABLE into output files (typically one
     or more 'Makefile's).  This means that 'AC_OUTPUT' will replace
     instances of '@VARIABLE@' in input files with the value that the
     shell variable VARIABLE has when 'AC_OUTPUT' is called.  The value
     of VARIABLE should not contain literal newlines.

 -- Macro: AC_SUBST_FILE (VARIABLE)
     Another way to create an output variable from a shell variable.
     Make 'AC_OUTPUT' insert (without substitutions) the contents of the
     file named by shell variable VARIABLE into output files.  This
     means that 'AC_OUTPUT' will replace instances of '@VARIABLE@' in
     output files (such as 'Makefile.in') with the contents of the file
     that the shell variable VARIABLE names when 'AC_OUTPUT' is called.
     Set the variable to '/dev/null' for cases that do not have a file
     to insert.

     This macro is useful for inserting 'Makefile' fragments containing
     special dependencies or other 'make' directives for particular host
     or target types into 'Makefile's.  For example, 'configure.in'
     could contain:

          AC_SUBST_FILE(host_frag)dnl
          host_frag=$srcdir/conf/sun4.mh

     and then a 'Makefile.in' could contain:

          @host_frag@