C99 compatibility fixes

[fweimer-rh]

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

[HaoLiHaiO]

I came here to fix this issue because I got it from the AUR on Arch Linux and timeout_init() doesn't compile anymore. It's been apparently been fixed but never merged.

[jonasob]

Good set of C99 compatibility fixes. All changes look correct:

1. arpa/inet.h detection and include — inet_pton is declared in <arpa/inet.h> on POSIX systems. Adding the AC_CHECK_HEADERS entry and the conditional include in the configure test is the right fix. Without this, implicit declaration of inet_pton is an error in C99+.

2. exit() → return in configure test main() — Avoids implicit declaration of exit() (which requires <stdlib.h>). Using return from main() is cleaner and doesn't need an extra include.

3. timeout_init() → timeout_init(void) — In C99+, f() means unspecified parameters (and is deprecated), while f(void) explicitly means no parameters. The matching declaration added to include/utils.h keeps the header and implementation in sync.

All three changes are minimal, correct, and necessary for building with modern C compilers (GCC 14+, Clang 16+) that default to C17/C23 where implicit function declarations and implicit ints are errors.

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