Build error with clang 16 on Alpine

[kenballus@…]

Nginx's current default branch (revision hash 44536076405cf79ebdd82a6a0ab27bb3aed86b04) fails to build with clang on Alpine Linux because of Werror and Wsign-compare.

Steps to reproduce:

0. Get a fresh Alpine Linux container:

docker run --rm --workdir /repro -it alpine:3.18.0

1. Get dependencies:

apk add mercurial clang musl-dev pcre-dev zlib-dev make

2. Clone nginx:

hg clone "https://hg.nginx.org/nginx"

3. Configure:

cd nginx && ./auto/configure --with-cc=clang

4. Try to build:

make

5. Observe the warning->error that halts the build:

   make -f objs/Makefile
   make[1]: Entering directory '/repro/nginx'
   ...
   clang -c -pipe  -O -Wall -Wextra -Wpointer-arith -Wconditional-uninitialized -Wno-unused-parameter -Werror -g  -I src/core -I src/event -I src/event/modules -I src/event/quic -I src/os/unix -I objs \
   	-o objs/src/event/ngx_event_udp.o \
   	src/event/ngx_event_udp.c
   src/event/ngx_event_udp.c:143:25: error: comparison of integers of different signs: 'unsigned long' and 'long' [-Werror,-Wsign-compare]
                    cmsg = CMSG_NXTHDR(&msg, cmsg))
                           ^~~~~~~~~~~~~~~~~~~~~~~
   /usr/include/sys/socket.h:358:44: note: expanded from macro 'CMSG_NXTHDR'
           __CMSG_LEN(cmsg) + sizeof(struct cmsghdr) >= __MHDR_END(mhdr) - (unsigned char *)(cmsg) \
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   1 error generated.
   make[1]: *** [objs/Makefile:641: objs/src/event/ngx_event_udp.o] Error 1
   make[1]: Leaving directory '/repro/nginx'
   make: *** [Makefile:10: build] Error 2
   

The build succeeds with GCC 12.2.1 on Alpine, and with clang 16 on Debian. Its only clang 16 on Alpine that fails.