build: check MSG_NOSIGNAL directly, drop detection and interim macro

Drop detecting it at configure time, along with the interim macro
`HAVE_MSG_NOSIGNAL`. There is no longer a reason for this workaround,
and allows to save the work at configure time and simplify.

Also say in a comment that `sys/socket.h` is defining this macro.

Follow-up to 77b3bc239d

Closes #20559

lib/curl_config-cmake.h.in

@@ -400,9 +400,6 @@
 /* Define to 1 if you have the 'suseconds_t' data type. */
 #cmakedefine HAVE_SUSECONDS_T 1
 
-/* Define to 1 if you have the MSG_NOSIGNAL flag. */
-#cmakedefine HAVE_MSG_NOSIGNAL 1
-
 /* Define to 1 if you have the <netdb.h> header file. */
 #cmakedefine HAVE_NETDB_H 1
 

lib/curl_setup.h

@@ -852,7 +852,7 @@
 #endif
 
 #ifndef _WIN32
-#include <sys/socket.h>
+#include <sys/socket.h>  /* also for MSG_NOSIGNAL */
 #endif
 
 #include "functypes.h"
@@ -880,7 +880,7 @@ struct timeval {
  * If we have the MSG_NOSIGNAL define, make sure we use
  * it as the fourth argument of function send()
  */
-#ifdef HAVE_MSG_NOSIGNAL
+#ifdef MSG_NOSIGNAL
 #define SEND_4TH_ARG MSG_NOSIGNAL
 #else
 #define SEND_4TH_ARG 0

lib/multi.c

@@ -1852,7 +1852,7 @@ static void set_in_callback(struct Curl_multi *multi, bool value)
  */
 static void multi_posttransfer(struct Curl_easy *data)
 {
-#if defined(HAVE_SIGNAL) && defined(SIGPIPE) && !defined(HAVE_MSG_NOSIGNAL)
+#if defined(HAVE_SIGNAL) && defined(SIGPIPE) && !defined(MSG_NOSIGNAL)
   /* restore the signal handler for SIGPIPE before we get back */
   if(!data->set.no_signal)
     signal(SIGPIPE, data->state.prev_signal);

lib/transfer.c

@@ -535,7 +535,7 @@ CURLcode Curl_pretransfer(struct Curl_easy *data)
      * different ports! */
     data->state.allow_port = TRUE;
 
-#if defined(HAVE_SIGNAL) && defined(SIGPIPE) && !defined(HAVE_MSG_NOSIGNAL)
+#if defined(HAVE_SIGNAL) && defined(SIGPIPE) && !defined(MSG_NOSIGNAL)
     /*************************************************************
      * Tell signal handler to ignore SIGPIPE
      *************************************************************/