Build failure with GCC 15 (defaults to -std=gnu23)

[thesamesam]

Upcoming GCC 15 defaults to -std=gnu23. gpm fails to build as follows:

lib/libcurses.c:46:5: error: conflicting types for ‘Gpm_Wgetch’; have ‘int(WINDOW *)’ {aka ‘int(struct _win_st *)’}
   46 | int Gpm_Wgetch(WINDOW *win)
      |     ^~~~~~~~~~
In file included from /var/tmp/portage/sys-libs/gpm-1.20.7-r6/work/gpm-1.20.7-abi_x86_32.x86/src/headers/gpmInt.h:28,
                 from lib/libcurses.c:30:
/var/tmp/portage/sys-libs/gpm-1.20.7-r6/work/gpm-1.20.7-abi_x86_32.x86/src/headers/gpm.h:271:12: note: previous declaration of ‘Gpm_Wgetch’ with type ‘int(void)’
  271 | extern int Gpm_Wgetch();
      |            ^~~~~~~~~~

C23 removes unprototyped functions.

It should be possible to reproduce these with older GCC (and Clang) with
-std=gnu23 or -std=c23 set manually.