configure.info: Write acconfig.h

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Write acconfig.h

   If you are generating a portability header file, (i.e., you are using
`AM_CONFIG_HEADER' in `configure.in'), then you will have to write a
`acconfig.h' file.  It will have to contain the following lines.
     /* Name of package.  */
     #undef PACKAGE
     /* Version of package.  */
     #undef VERSION
   This requirement is really a bug in the system, and the requirement
may be eliminated at some later date.
   The `acconfig.h' file will also similar comment and `#undef' lines
for any unusual macros in the `configure.in' file, including any macro
which appears in a `AC_DEFINE' macro.
   In particular, if you are writing a GNU package and therefore include
`AC_DEFINE(_GNU_SOURCE)' in `configure.in' as suggested above, you will
need lines like this in `acconfig.h':
     /* Enable GNU extensions.  */
     #undef _GNU_SOURCE
   Normally the `autoheader' program will inform you of any such
requirements by printing an error message when it is run.  However, if
you do anything particular odd in your `configure.in' file, you will
have to make sure that the right entries appear in `acconfig.h', since
otherwise the results of the tests may not be available in the
`config.h' file which your code will use.
   (Thee `PACKAGE' and `VERSION' lines are not required if you are not
using automake, and in that case you may not need a `acconfig.h' file
at all).