Hi.

This patch addresses one of the differences between 'fontconfig' 
font-weight table, and the one inside emacs. Basically, the problem was 
that emacs wasn't listing fonts whose weight=55. However, those are 
defined to be 'semilight' in the fontconfig specification (see link below).

Reproducing this problem only requires the existence of fonts whose 
weight=55 (one can use fc-cat for that). It's possible to observe that 
such fonts are not found in 'x-list-fonts' or when running 'describe-font'.

https://www.freedesktop.org/software/fontconfig/fontconfig-user.html

I don't fully understand how emacs is using the 'weight_table' (in 
font.c), so my explanation may not be fully correct, but this patch does 
make (fontconfig) semilight fonts show up.

Regards,

Yukio