host.c: print_backtrace: glibc <2.27 compat

CentOS 7 has glibc 2.17 not supporting memfd_create, so use the generic
POSIX solution as for macOS. Also switched the condition to this be used
always but when glibc 2.27+ was not detected.

src/host.cpp

@@ -1059,14 +1059,14 @@ print_backtrace()
 #ifndef _WIN32
         // print to a temporary file to avoid interleaving from multiple
         // threads
-#ifdef __APPLE__
+#ifdef __GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 27)
+        int fd = memfd_create("ultragrid_backtrace", MFD_CLOEXEC);
+#else
         char path[MAX_PATH_SIZE];
         snprintf(path, sizeof path, "%s/ug-%u", get_temp_dir(),
                  pthread_mach_thread_np(pthread_self()));
         int fd = open(path, O_CLOEXEC | O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
         unlink(path);
-#else
-        int fd = memfd_create("ultragrid_backtrace", MFD_CLOEXEC);
 #endif
         if (fd == -1) {
                 fd = STDERR_FILENO;