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

CMake/OtherTests.cmake

@@ -50,19 +50,6 @@ if(NOT DEFINED HAVE_STRUCT_SOCKADDR_STORAGE)
   cmake_pop_check_state()
 endif()
 
-if(NOT WIN32)
-  set(_source_epilogue "#undef inline")
-  curl_add_header_include(HAVE_SYS_TYPES_H "sys/types.h")
-  check_c_source_compiles("${_source_epilogue}
-    #include <sys/socket.h>
-    int main(void)
-    {
-      int flag = MSG_NOSIGNAL;
-      (void)flag;
-      return 0;
-    }" HAVE_MSG_NOSIGNAL)
-endif()
-
 set(_source_epilogue "#undef inline")
 check_c_source_compiles("${_source_epilogue}
   #ifdef _MSC_VER

CMake/unix-cache.cmake

@@ -179,7 +179,6 @@ if(APPLE OR
 else()
   set(HAVE_MEMRCHR 1)
 endif()
-set(HAVE_MSG_NOSIGNAL 1)
 set(HAVE_NETDB_H 1)
 if(ANDROID)
   set(HAVE_NETINET_IN6_H 1)

CMake/win32-cache.cmake

@@ -125,7 +125,6 @@ set(HAVE_LINUX_TCP_H 0)
 set(HAVE_LOCALE_H 1)
 set(HAVE_LOCALTIME_R 0)
 set(HAVE_MEMRCHR 0)
-set(HAVE_MSG_NOSIGNAL 0)
 set(HAVE_NETDB_H 0)
 set(HAVE_NETINET_IN6_H 0)
 set(HAVE_NETINET_IN_H 0)