Fix warnings/issues from CI/CT

VisualStudio/unit_test/unit_test.vcxproj

@@ -73,7 +73,7 @@
       </PrecompiledHeader>
       <WarningLevel>Level3</WarningLevel>
       <Optimization>Disabled</Optimization>
-      <PreprocessorDefinitions>MAIN_PUBLIC_SERVER=main;NO_SSL;LOCAL_TEST;REPLACE_CHECK_FOR_LOCAL_DEBUGGING;LOCAL_TEST;USE_IPV6;USE_WEBSOCKET;MEMORY_DEBUGGING;WIN32;_DEBUG;_CONSOLE;_CRT_SECURE_NO_WARNINGS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>MAIN_PUBLIC_SERVER=main;OPENSSL_API_1_1;LOCAL_TEST;REPLACE_CHECK_FOR_LOCAL_DEBUGGING;LOCAL_TEST;USE_IPV6;USE_WEBSOCKET;MEMORY_DEBUGGING;WIN32;_DEBUG;_CONSOLE;_CRT_SECURE_NO_WARNINGS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <AdditionalIncludeDirectories>$(ProjectDir)..\..\src;$(ProjectDir)..\..\include;$(ProjectDir)..\..\src\third_party\lua-5.2.4\src;$(ProjectDir)..\..\..\check-0.10.0\;$(ProjectDir)..\..\..\check-0.10.0\src\;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
     </ClCompile>
     <Link>

src/civetweb.c

@@ -1339,7 +1339,7 @@ mg_atomic_add(volatile ptrdiff_t *addr, ptrdiff_t value)
 
 
 FUNCTION_MAY_BE_UNUSED
-static int
+static ptrdiff_t
 mg_atomic_compare_and_swap(volatile ptrdiff_t *addr,
                            ptrdiff_t oldval,
                            ptrdiff_t newval)

src/response.inl

@@ -249,7 +249,6 @@ int
 mg_response_header_send(struct mg_connection *conn)
 {
 #if !defined(NO_RESPONSE_BUFFERING)
-	const char *txt;
 	int i;
 	int has_date = 0;
 	int has_connection = 0;

unittest/public_server.c

@@ -807,6 +807,14 @@ START_TEST(test_mg_server_and_client_tls)
 		ck_assert_int_lt(client_res, 0); /* response is "error" (-1) */
 		ck_assert_str_ne(client_err, "");
 		client_ri = mg_get_response_info(client_conn);
+		if (client_ri) {
+			/* client_ri == NULL is allowed. However, some versions seem to
+			 * return non-null, but all elements are NULL. */
+			ck_assert_int_eq(client_ri->status_code, 0);
+			ck_assert_int_eq(client_ri->num_headers, 0);
+			ck_assert_ptr_eq(client_ri->http_version, NULL);
+			client_ri = NULL;
+		}
 		ck_assert(client_ri == NULL);
 
 		mg_close_connection(client_conn);