Use -std=gnu17 to avoid C23 errors

[shugo]

gcc -c -O4 -Wall -pedantic -I.. -DPDC_WIDE ../pdcurses/addch.c
In file included from ../curspriv.h:10,
                 from ../pdcurses/addch.c:3:
../curses.h:86:23: error: 'bool' cannot be defined via 'typedef'
   86 | typedef unsigned char bool;
      |                       ^~~~
../curses.h:86:23: note: 'bool' is a keyword with '-std=c23' onwards
../curses.h:86:1: warning: useless type name in empty declaration
   86 | typedef unsigned char bool;
      | ^~~~~~~