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Automake includes a number of Autoconf macros that can be used in
your package (see Autoconf macros supplied with Automake); some of them are actually required by
Automake in certain situations. These macros must be defined in your
aclocal.m4; otherwise they will not be seen by
autoconf.
The aclocal program will automatically generate
aclocal.m4 files based on the contents of configure.ac.
This provides a convenient way to get Automake-provided macros,
without having to search around. The aclocal mechanism
allows other packages to supply their own macros (see Writing your own aclocal macros). You can also use it to maintain your own set of custom
macros (see Handling Local Macros).
At startup, aclocal scans all the .m4 files it can
find, looking for macro definitions (see Macro Search Path). Then
it scans configure.ac. Any mention of one of the macros found
in the first step causes that macro, and any macros it in turn
requires, to be put into aclocal.m4.
Putting the file that contains the macro definition into
aclocal.m4 is usually done by copying the entire text of this
file, including unused macro definitions as well as both ‘#’ and
‘dnl’ comments. If you want to make a comment that will be
completely ignored by aclocal, use ‘##’ as the comment
leader.
When a file selected by aclocal is located in a subdirectory
specified as a relative search path with aclocal’s -I
argument, aclocal assumes the file belongs to the package
and uses m4_include instead of copying it into
aclocal.m4. This makes the package smaller, eases dependency
tracking, and cause the file to be distributed automatically.
(See Handling Local Macros, for an example.) Any macro that is found in a
system-wide directory, or via an absolute search path will be copied.
So use ‘-I `pwd`/reldir’ instead of ‘-I reldir’ whenever
some relative directory should be considered outside the package.
The contents of acinclude.m4, if this file exists, are also automatically included in aclocal.m4. We recommend against using acinclude.m4 in new packages (see Handling Local Macros).
While computing aclocal.m4, aclocal runs
autom4te (see Using Autom4te in The Autoconf Manual) in order to trace the macros that are
really used, and omit from aclocal.m4 all macros that are
mentioned but otherwise unexpanded (this can happen when a macro is
called conditionally). autom4te is expected to be in the
PATH, just as autoconf. Its location can be
overridden using the AUTOM4TE environment variable.