|  |  C.2 autoheader 
autoheaderrunsm4over `configure.in', but
with key macros defined differently than whenautoconfis
executed, such that suitablecppdefinitions are output to
`config.h.in'. 
 |  | user input files    optional input     process          output files
================    ==============     =======          ============
                    aclocal.m4 - - - - - - - .
                    (acconfig.h) - - - -.    |
                                        V    V
                                     .----------,
configure.in ----------------------->|autoheader|----> config.h.in
                                     `----------'
 | 
 
 |