As summary.
Why?
It appears to install fine but does give this warning: ... checking host system type... x86_64-pc-linux-gnu checking for a sed that does not truncate output... /bin/sed Package xorg-macros was not found in the pkg-config search path. Perhaps you should add the directory containing `xorg-macros.pc' to the PKG_CONFIG_PATH environment variable No package 'xorg-macros' found checking for RGB... yes ...
It's OK when native build. But fail when cross-compiling.
Fixed in rgb-1.0.6.