Dependency tracking is performed as a side-effect of compilation.
Each time the build system compiles a source file, it computes its
list of dependencies (in C these are the header files included by the
source being compiled). Later, any time make is run and a
dependency appears to have changed, the dependent files will be
rebuilt.
Automake generates code for automatic dependency tracking by default, unless the developer chooses to override it; for more information, see Automatic dependency tracking.
When configure is executed, you can see it probing each
compiler for the dependency mechanism it supports (several mechanisms
can be used):
~/amhello-1.0 % ./configure --prefix /usr ... checking dependency style of gcc... gcc3 ...
Because dependencies are only computed as a side-effect of the
compilation, no dependency information exists the first time a package
is built. This is OK because all the files need to be built anyway:
make does not have to decide which files need to be rebuilt.
In fact, dependency tracking is completely useless for one-time builds
and there is a configure option to disable this:
Speed up one-time builds.
Some compilers do not offer any practical way to derive the list of
dependencies as a side-effect of the compilation, requiring a separate
run (maybe of another tool) to compute these dependencies. The
performance penalty implied by these methods is important enough to
disable them by default. The option --enable-dependency-tracking
must be passed to configure to activate them.
Do not reject slow dependency extractors.
See Dependency Tracking Evolution in Brief History of Automake, for some discussion about the different dependency tracking schemes used by Automake over the years.