Unbreak build on macOS

[Mattias Engdegård]

On macOS, lib/fcntl.h does

#   define open rpl_open

which now (f8ab90839f) gets included in src/font.c and breaks the line

      font_object = driver_list->driver->open (f, entity, psize);

As an immediate remedy, I added

#undef open

to src/font.c.

In general, '#define open' is a rather unpleasant thing to do. Can we at least 
assume enough of C99 to do

#define open(...) rpl_open (__VA_ARGS__)

instead? That would permit font.c to use bracketing as an alternative 
countermeasure.