configure: Fix return values in start thread routines

Thread start routines must return a void * value, and future
compilers refuse to convert integers to pointers with just a warning
(the virtualtimer probe).  Without this change, the probe always fails
to compile with future compilers (such as GCC 14).

For the tls probe, return a null pointer for future-proofing, although
current and upcoming C compilers do not treat this omission as an
error.

Updates commit dd11311 ("configure: fix tls detection").

src/configure

@@ -5250,6 +5250,7 @@ else
 		                       res1 = (i == (int)arg);
                              else
 		                       res2 = (i == (int)arg);
+		             return NULL;
                            }
 	                       __thread int i;
                            int main () {
@@ -5378,7 +5379,7 @@ else
                                exit(1);
                              }
                              done = 1;
-	                         return j;
+			     return (void *) j;
                            }
 
                            int main( int argc, char ** argv ) {

src/configure.in

@@ -721,6 +721,7 @@ AC_ARG_WITH(tls,
 		                       res1 = (i == (int)arg);
                              else
 		                       res2 = (i == (int)arg);
+		             return NULL;
                            }
 	                       __thread int i;
                            int main () {
@@ -812,7 +813,7 @@ AC_ARG_WITH(virtualtimer,
                                exit(1);
                              }        
                              done = 1;
-	                         return j;
+			     return (void *) j;
                            } 
 
                            int main( int argc, char ** argv ) {