Remove redundant checks for MSG_* from configure.ac

It is redundant to check for the presence of MSG_NOSIGNAL macro in
configure.ac, define HAVE_MSG_NOSIGNAL and then check whether the later
is defined in the source code. Instead we can check directly whether
MSG_NOSIGNAL is defined. Same for MSG_DONTWAIT.

In addition to that, the checks we had in configure.ac produce a
compiler warning about unused variable and thus could fail if
-Werror is present and erroneously proclaim that the macros are
not available.

src/net.cpp

@@ -42,12 +42,13 @@
 // We add a random period time (0 to 1 seconds) to feeler connections to prevent synchronization.
 #define FEELER_SLEEP_WINDOW 1
 
-#if !defined(HAVE_MSG_NOSIGNAL)
+// MSG_NOSIGNAL is not available on some platforms, if it doesn't exist define it as 0
+#if !defined(MSG_NOSIGNAL)
 #define MSG_NOSIGNAL 0
 #endif
 
 // MSG_DONTWAIT is not available on some platforms, if it doesn't exist define it as 0
-#if !defined(HAVE_MSG_DONTWAIT)
+#if !defined(MSG_DONTWAIT)
 #define MSG_DONTWAIT 0
 #endif
 

src/netbase.cpp

@@ -21,7 +21,7 @@
 #include <boost/algorithm/string/case_conv.hpp> // for to_lower()
 #include <boost/algorithm/string/predicate.hpp> // for startswith() and endswith()
 
-#if !defined(HAVE_MSG_NOSIGNAL)
+#if !defined(MSG_NOSIGNAL)
 #define MSG_NOSIGNAL 0
 #endif