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The configmake module builds a C include file named
configmake.h containing the usual installation directory
values; for example, those specified by --prefix or
--libdir to configure. Each variable is given a #define
with an all-uppercase macro name, such as PREFIX and
LIBDIR. (Automake cannot create this file directly because the
user might override directory values at make time.)
Specifically, the module retrieves values of the variables through
configure followed by make, not directly through
configure, so that a user who sets some of these variables
consistently on the make command line gets correct results.
One advantage of this approach, compared to the classical approach of
adding -DLIBDIR=\"$(libdir)\" etc. to AM_CPPFLAGS, is
that it protects against the use of undefined variables. That is, if,
say, $(libdir) is not set in the Makefile, LIBDIR is not
defined by this module, and code using LIBDIR gives a
compilation error.
Another advantage is that make output is shorter.
For the complete list of variables which are #defined this way,
see the file gnulib/modules/configmake, or inspect your
resulting gnulib Makefile.