Fix build on musl libc with clang.

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>

src/nxt_socket_msg.h

@@ -69,6 +69,20 @@ NXT_EXPORT ssize_t nxt_recvmsg(nxt_socket_t s,
     nxt_iobuf_t *iob, nxt_uint_t niob, nxt_recv_oob_t *oob);
 
 
+nxt_inline struct cmsghdr *
+NXT_CMSG_NXTHDR(struct msghdr *msgh, struct cmsghdr *cmsg)
+{
+#if !defined(__GLIBC__) && defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wsign-compare"
+#endif
+    return CMSG_NXTHDR(msgh, cmsg);
+#if !defined(__GLIBC__) && defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+}
+
+
 nxt_inline void
 nxt_socket_msg_oob_init(nxt_send_oob_t *oob, int *fds)
 {
@@ -135,7 +149,7 @@ nxt_socket_msg_oob_get_fds(nxt_recv_oob_t *oob, nxt_fd_t *fd)
 
     for (cmsg = CMSG_FIRSTHDR(&msg);
          cmsg != NULL;
-         cmsg = CMSG_NXTHDR(&msg, cmsg))
+         cmsg = NXT_CMSG_NXTHDR(&msg, cmsg))
     {
         size = cmsg->cmsg_len - CMSG_LEN(0);
 
@@ -174,7 +188,7 @@ nxt_socket_msg_oob_get(nxt_recv_oob_t *oob, nxt_fd_t *fd, nxt_pid_t *pid)
 
     for (cmsg = CMSG_FIRSTHDR(&msg);
          cmsg != NULL;
-         cmsg = CMSG_NXTHDR(&msg, cmsg))
+         cmsg = NXT_CMSG_NXTHDR(&msg, cmsg))
     {
         size = cmsg->cmsg_len - CMSG_LEN(0);