Needed to avoid error in case xxd is not installed:
[ 16%] Generating html.h from index.html
/bin/sh: 1: CMAKE_XXD-NOTFOUND: not found
CMakeFiles/ttyd.dir/build.make:61: recipe for target 'html.h' failed
make[6]: *** [html.h] Error 127
Signed-off-by: Rosen Penev <rosenp@gmail.com>