util: gettid() is Linux-specific

The manual page clearly states that

  gettid() is Linux-specific and should not be used in programs
  that are intended to be portable.

Unfortunately, it looks like macOS implemented the functionality
and defined SYS_gettid accordingly, only to deprecate syscall()
altogether with 10.12 (Sierra), released last late year.

To avoid compilation errors, call gettid() on Linux only.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>

src/util/virthread.c

@@ -268,7 +268,7 @@ bool virThreadIsSelf(virThreadPtr thread)
  * the pthread_self() id on Linux.  */
 unsigned long long virThreadSelfID(void)
 {
-#if defined(HAVE_SYS_SYSCALL_H) && defined(SYS_gettid)
+#if defined(HAVE_SYS_SYSCALL_H) && defined(SYS_gettid) && defined(__linux__)
     pid_t tid = syscall(SYS_gettid);
     return tid;
 #else