Automake includes a number of Autoconf macros which can be used in your
package; 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.in. This provides a
convenient way to get Automake-provided macros, without having to
search around. Also, the aclocal mechanism allows other packages
to supply their own macros.
At startup, aclocal scans all the .m4 files it can find,
looking for macro definitions (see Macro search path). Then it
scans configure.in. 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.
The contents of acinclude.m4, if it exists, are also
automatically included in aclocal.m4. This is useful for
incorporating local macros into configure.
aclocal tries to be smart about looking for new AC_DEFUNs
in the files it scans. It also
tries to copy the full text of the scanned file into aclocal.m4,
including both # and dnl comments. If you want to make a
comment which will be completely ignored by aclocal, use
## as the comment leader.