Fix redefinition of inet_pton with mingw-w64

It may be missing in mingw32, but not in mingw-w64.
But travis is on Ubuntu Trusty, so its mingw-w64 is ancient and missing inet_pton,
so the proper thing to do is to check for it.

CMakeLists.txt

@@ -19,6 +19,7 @@ include(haiUtil)
 include(FindPkgConfig)
 # XXX See 'if (MINGW)' condition below, may need fixing.
 include(FindThreads)
+include(CheckFunctionExists)
 
 # Platform shortcuts
 set_if(DARWIN      ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
@@ -87,6 +88,7 @@ option(ENABLE_GETNAMEINFO "In-logs sockaddr-to-string should do rev-dns" OFF)
 option(USE_GNUTLS "Should use gnutls instead of openssl" OFF)
 option(ENABLE_CXX_DEPS "Extra library dependencies in srt.pc for the CXX libraries useful with C language" ON)
 option(USE_STATIC_LIBSTDCXX "Should use static rather than shared libstdc++" OFF)
+option(ENABLE_INET_PTON "Set to OFF to prevent usage of inet_pton when building against modern SDKs while still requiring compatibility with older Windows versions, such as Windows XP, Windows Server 2003 etc." ON)
 
 set(TARGET_srt "srt" CACHE STRING "The name for the haisrt library")
 
@@ -145,6 +147,22 @@ endif()
 # When you use crosscompiling, you have to take care that PKG_CONFIG_PATH
 # and CMAKE_PREFIX_PATH are set properly.
 
+# symbol exists in win32, but function does not.
+if(WIN32)
+	if(ENABLE_INET_PTON)
+		set(CMAKE_REQUIRED_LIBRARIES ws2_32)
+		check_function_exists(inet_pton HAVE_INET_PTON)
+		add_definitions(-D_WIN32_WINNT=0x0600)
+	else()
+		add_definitions(-D_WIN32_WINNT=0x0501)
+	endif()
+else()
+	check_function_exists(inet_pton HAVE_INET_PTON)
+endif()
+if (DEFINED HAVE_INET_PTON)
+	add_definitions(-DHAVE_INET_PTON=1)
+endif()
+
 # find OpenSSL
 if ( USE_GNUTLS )
 	message(STATUS "SSL Dependency: using GNUTLS with Nettle, as requested")

apps/apputil.hpp

@@ -76,7 +76,7 @@ inline void SysCleanupNetwork() {}
 // See:
 //    https://msdn.microsoft.com/en-us/library/windows/desktop/ms742214(v=vs.85).aspx
 //    http://www.winsocketdotnetworkprogramming.com/winsock2programming/winsock2advancedInternet3b.html
-#ifdef __MINGW32__
+#if defined(_WIN32) && !defined(HAVE_INET_PTON)
 static inline int inet_pton(int af, const char * src, void * dst)
 {
    struct sockaddr_storage ss;
@@ -115,7 +115,7 @@ static inline int inet_pton(int af, const char * src, void * dst)
 
    return 0;
 }
-#endif // __MINGW__
+#endif // _WIN32 && !HAVE_INET_PTON
 
 #ifdef _WIN32
 inline int SysError() { return ::GetLastError(); }