o Concocted a "ACX_CONFIG_PREFIXED_HEADER" macro, to create an installable
config header (mjpegtools-config.h -- not used yet)
o Removed extraneous AC_SUBSTS (that are already created within macros).
o Re-instate "HAVE_LIBDV" ac_define, which was accidentally axed earlier.
o Changed "dv", "quicktime", "png" to be consistently "libdv", "libquicktime",
and "libpng".
o Account for those changes in 'lavtools/Makefile.am'.
o A little modernization of 'lavtools/Makefile.am'
