The C bindings, top level MPI C functions, will be generated by
`maint/gen_binding_c.py` during autogen to this folder along with
Makefile.mk.

For a function to be generated, it has to be listed in one of the
"config" file named in patterns of dir_api.txt. For example,
pt2pt_api.txt should list all mpi functions to be generated to the
pt2pt folders.

The script, gen_binding_c.py will preload function information from
master file, maint/mpi_standard_api.txt, so the config file here
only need list minimum amount of extra information. For example, an
entry of 
    MPI_Sendrecv:
        .desc: Sends and receive a message
will generate the entire `pt2pt/sendrecv.c`. However, if desired,
one can use the config file to overwrite any information in the 
master file. For MPIX_ functions, since it is missing from the master
file, everything include all the parameter information have to be
specified in the config file, or the generated file will be incomplete.

A function exist in the master file but not listed in one of the
config file here will not be generated.
