example) the AC_SYS_LARGEFILE check determines that nothing special needs
to be done for supporting large files - this places "/* #undef _FILE_OFFSET_BITS */"
in config.h.
Changes were made that if NO size limit was explicitly given (usually '-m #MB'
for the tools) then allow unlimited filesize (the ODML extensions will handle
the AVI files and Quicktime hasn't had a size limit in eons).
Also, consolidate the 3 copies of the size limit definitions into one
place (lav_io.h).
