Log native thread ID when a thread is created.

If possible gettid() is used, otherwise pthread_self() is logged (which is
not completely portable, but seems to work on most supported platforms).

configure.in

@@ -380,6 +380,17 @@ dnl check if native rwlocks are available
 AC_CHECK_FUNCS(pthread_rwlock_init)
 LIBS=$saved_LIBS
 
+AC_MSG_CHECKING([for gettid])
+AC_TRY_COMPILE(
+	[#define _GNU_SOURCE
+	#include <unistd.h>
+	#include <sys/syscall.h>],
+	[int main() {
+		return syscall(SYS_gettid);}],
+	[AC_MSG_RESULT([yes]); AC_DEFINE([HAVE_GETTID])],
+	[AC_MSG_RESULT([no])]
+)
+
 AC_CHECK_FUNCS(prctl mallinfo getpass closefrom)
 
 AC_CHECK_HEADERS(sys/sockio.h glob.h)