acconfig.h is used to generate config.h.in, which is the
file used to generate config.h.
It should contain all the macro definitions needed to control the program.
In it, a line of #undef MYMACRO specifies that the macro should
not be defined by default. If it was defined during the run of the ./configure
script, it would replace it as such.
A line containing #define MYMACRO specifies that the macro would
always be defined, but its value may vary.
The utility autoheader can help in creating an initial
config.h.in file.