old-makefile: add a missing source directory

Fixes "make clean".
player: don't complain on too long filenames

mpv supports per-file config files, basically filename+".conf". We use
a static buffer for the new filename, and if that buffer is too small,
we print a warning. This is confusing for e.g. long URLs, so just hide
the warning by default.

Why not dynamically allocate the buffer? Who cares.