Rename makefile variable

There are many "products" per subdirectory.
Have color-io in own library

"real.h" can be, and is, used for other purposes as well,
namely a application-global type for a selectabe floating-point type.
Add pngquant as prerequisite in readme
Generate silent video if lame is not installed

The script will output the err of "type lame"
and in any case (also missing raw audio file, 'silent'.
Argument interface to cfluid