Writing a table generator

This documentation is preliminary.
Parts of the API are not good and should be changed.

Basic concepts

A table generator consists of two files, *_tablegen.c and *_tablegen.h.
The .h file will provide the variable declarations and initialization
code for the tables, the .c describes the tables so they can be printed
as a header file.
Both of these files will be compiled for the host system, so to avoid
breakage with cross-compilation neither of them may include, directly
or indirectly, config.h or avconfig.h.
Due to this, the .c file or Makefile may have to provide additional defines
or stubs, though if possible this should be avoided.

The .c file

This file should include the *_tablegen.h and tableprint.h files and
anything else it needs as long as it does not depend on config.h or
avconfig.h.
In addition to that it must contain a void tableinit(void) function
which initializes all tables by calling the init functions from the .h
file.
It must also contain a "const struct tabledef tables[]" array describing
the tables to be generated.
Its entries consist of (in order):
 - a string suitable for declaring the table, up to but not including the =
   NULL terminates the table
 - a function to print the table - tableprint.h defines some defaults,
   e.g. write_uint8_array to print a uint8_t array.
 - a pointer to the table
 - the size of the first dimension of the array
 - if applicable, the size of the second dimension of the array

The .h file

This file should contain:
 - one or more initialization functions
 - the table variable declarations
If CONFIG_HARDCODED_TABLES is set, the initialization functions should
not do anything, and instead of the variable declarations the
generated *_tables.h file should be included.
Since that will be generated in the build directory, the path must be
included, i.e.
#include "libavcodec/example_tables.h"
not
#include "example_tables.h"

Makefile changes

To make the automatic table creation work, you must manually declare the
new dependency.
For this add a line similar to this:
$(SUBDIR)example.o: $(SUBDIR)example_tables.h
under the "ifdef CONFIG_HARDCODED_TABLES" section in the Makefile.