Automake works by reading a Makefile.am and generating a
Makefile.in. Certain variables and targets defined in the
Makefile.am instruct Automake to generate more specialized code;
for instance, a bin_PROGRAMS variable definition will cause targets
for compiling and linking programs to be generated.
The variable definitions and targets in the Makefile.am are copied
verbatim into the generated file. This allows you to add arbitrary code
into the generated Makefile.in. For instance the Automake
distribution includes a non-standard cvs-dist target, which the
Automake maintainer uses to make distributions from his source control
system.
Note that most GNU make extensions are not recognized by Automake. Using
such extensions in a Makefile.am will lead to errors or confusing
behavior.
A special exception is that the GNU make append operator, +=, is
supported. This operator appends its right hand argument to the variable
specified on the left. Automake will translate the operator into
an ordinary = operator; += will thus work with any make program.
Automake tries to keep comments grouped with any adjoining targets or variable definitions.
A target defined in Makefile.am generally overrides any such
target of a similar name that would be automatically generated by
automake. Although this is a supported feature, it is generally
best to avoid making use of it, as sometimes the generated rules are
very particular.
Similarly, a variable defined in Makefile.am or AC_SUBST'ed
from configure.in will override any definition of the variable that
automake would ordinarily create. This feature is more often
useful than the ability to override a target definition. Be warned that
many of the variables generated by automake are considered to be for
internal use only, and their names might change in future releases.
When examining a variable definition, Automake will recursively examine
variables referenced in the definition. For example, if Automake is
looking at the content of foo_SOURCES in this snippet
xs = a.c b.c
foo_SOURCES = c.c $(xs)
it would use the files a.c, b.c, and c.c as the
contents of foo_SOURCES.
Automake also allows a form of comment which is not copied into
the output; all lines beginning with ## (leading spaces allowed)
are completely ignored by Automake.
It is customary to make the first line of Makefile.am read:
## Process this file with automake to produce Makefile.in