gcc-15 switched to -std=c23 by default:

    https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=55e3bd376b2214e200fa76d12b67ff259b06c212

As a result `ncompress` fails the build as:

    compress.c: In function 'main':
    compress.c:382:40: error:
      passing argument 2 of 'signal' from incompatible pointer type [-Wincompatible-pointer-types]
      382 |                         signal(SIGINT, (SIG_TYPE)abort_compress);
          |                                        ^~~~~~~~~~~~~~~~~~~~~~~~
          |                                        |
          |                                        void (*)(void)
    In file included from compress.c:30:
    ...-glibc-2.40-36-dev/include/signal.h:88:57: note:
      expected '__sighandler_t' {aka 'void (*)(int)'} but argument is of type 'void (*)(void)'
       88 | extern __sighandler_t signal (int __sig, __sighandler_t __handler)
          |                                          ~~~~~~~~~~~~~~~^~~~~~~~~

The change removes type casts around function prototypes and define
signal handler as `void(*)(int)`.