Fix server for external test

For the client unit test, we need to contact some "non-civetweb" web server.
Since we cannot host one in all test container environments, use an external web server,
e.g., the server hosting the project.
We will only make two "GET" requests to these servers, so the traffic is negligible.

unittest/public_server.c

@@ -41,6 +41,10 @@
 #include <windows.h>
 #define test_sleep(x) (Sleep((x)*1000))
 #else
+#include <arpa/inet.h>
+#include <netdb.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
 #include <unistd.h>
 #define test_sleep(x) (sleep(x))
 #endif
@@ -52,7 +56,48 @@
 #define SLEEP_BEFORE_MG_STOP (1)
 #define SLEEP_AFTER_MG_STOP (5)
 
-static const char *external_server_ip = "140.82.118.4"; /* github.com */
+
+/* Try to communicate with an external http server. */
+static const char *
+get_external_server_ip(void)
+{
+#define no_of_testhosts (4)
+	const char *testhost[no_of_testhosts] = {"github.com",
+	                                         "google.com",
+	                                         "sourceforge.net",
+	                                         "microsoft.com"};
+	static char external_ip[64] = {0};
+	int testhostidx;
+
+	if (external_ip[0]) {
+		/* Get IP of external server only once, then reuse it */
+		return external_ip;
+	}
+	mark_point();
+
+	for (testhostidx = 0; testhostidx < no_of_testhosts; testhostidx++) {
+		struct hostent *hostentry;
+		struct in_addr **in_addr_list;
+		const char *ip;
+
+		hostentry = gethostbyname(testhost[testhostidx]);
+
+		if (hostentry != NULL) {
+			in_addr_list = (struct in_addr **)hostentry->h_addr_list;
+			if (in_addr_list[0] != NULL) {
+				ip = inet_ntoa(*in_addr_list[0]);
+				if (ip != NULL) {
+					mark_point();
+					strcpy(external_ip, ip);
+					return external_ip;
+				}
+			}
+		}
+	}
+
+	ck_abort_msg("Could not determine IP of any external server");
+	return "0.0.0.0";
+}
 
 
 /* This unit test file uses the excellent Check unit testing library.
@@ -4822,6 +4867,9 @@ minimal_https_client_check(const char *server,
 
 START_TEST(test_minimal_client)
 {
+	const char *external_server_ip;
+	mark_point();
+	external_server_ip = get_external_server_ip();
 	mark_point();
 
 	/* Initialize the library */
@@ -4847,6 +4895,9 @@ END_TEST
 
 START_TEST(test_minimal_tls_client)
 {
+	const char *external_server_ip;
+	mark_point();
+	external_server_ip = get_external_server_ip();
 	mark_point();
 
 #if !defined(NO_SSL) /* dont run https test if SSL is not enabled */