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(autoconf.info)New Macros

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New Macros
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   Because Autoconf has been dormant for years, Automake provided
Autoconf-like macros for a while.  Autoconf 2.50 now provides better
versions of these macros, integrated in the `AC_' namespace, instead of
`AM_'.  But in order to ease the upgrading via `autoupdate', bindings
to such `AM_' macros are provided.

   Unfortunately Automake did not quote the name of these macros!
Therefore, when `m4' finds something like `AC_DEFUN(AM_TYPE_PTRDIFF_T,
...)' in `aclocal.m4', `AM_TYPE_PTRDIFF_T' is expanded, replaced with
its Autoconf definition.

   Fortunately Autoconf catches pre-`AC_INIT' expansions, and will
complain, in its own words:

     $ cat configure.in
     AC_INIT
     AM_TYPE_PTRDIFF_T
     $ aclocal-1.4
     $ autoconf
     ./aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion
     actypes.m4:289: AM_TYPE_PTRDIFF_T is expanded from...
     ./aclocal.m4:17: the top level
     $

   Future versions of Automake will simply no longer define most of
these macros, and will properly quote the names of the remaining macros.
But you don't have to wait for it to happen to do the right thing right
now: do not depend upon macros from Automake as it is simply not its job
to provide macros (but the one it requires by itself):

     $ cat configure.in
     AC_INIT
     AM_TYPE_PTRDIFF_T
     $ rm aclocal.m4
     $ autoupdate
     autoupdate: `configure.in' is updated
     $ cat configure.in
     AC_INIT
     AC_CHECK_TYPES([ptrdiff_t])
     $ aclocal-1.4
     $ autoconf
     $