As reported by @andypost on GitHub, if you try to build Unit on a system
that uses musl libc (such as Alpine Linux) with clang then you get the
following

clang -c -pipe -fPIC -fvisibility=hidden -O -W -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -fstrict-aliasing -Wstrict-overflow=5 -Wmissing-prototypes -Werror -g   -I src -I build/include   \
                      \
                     \
-o build/src/nxt_socketpair.o \
-MMD -MF build/src/nxt_socketpair.dep -MT build/src/nxt_socketpair.o \
src/nxt_socketpair.c
In file included from src/nxt_socketpair.c:8:
src/nxt_socket_msg.h:138:17: 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) \
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from src/nxt_socketpair.c:8:
src/nxt_socket_msg.h:177:17: 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) \
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.
make: *** [build/Makefile:261: build/src/nxt_socketpair.o] Error 1

GCC works fine, it seems to have some smarts so that it doesn't give
warnings on system header files.

This seems to be a long standing issue with musl libc (bad casting in
the CMSG_NXTHDR macro) and the workaround employed by several projects
is to disable the -Wsign-compare clang warning for the code in question.

So, that's what we do. We wrap the CMSG_NXTHDR macro in a function, so
we can use the pre-processor in it to selectively disable the warning.

Link: <https://github.com/dotnet/runtime/issues/16438>
Link: <https://git.openembedded.org/meta-openembedded/tree/meta-oe/recipes-devtools/breakpad/breakpad/0001-Turn-off-sign-compare-for-musl-libc.patch>
Link: <https://github.com/dotnet/corefx/blob/57ff88bb75a0/src/Native/Unix/System.Native/pal_networking.c#L811-L829>
Link: <https://patchwork.yoctoproject.org/project/oe/patch/20220407191438.3696227-1-stefan@datenfreihafen.org/>
Closes: <https://github.com/nginx/unit/issues/936>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>