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.

Submitted upstream: <https://github.com/icl-utk-edu/papi/pull/142>

diff --git a/src/configure b/src/configure
index 8ce0047d803bd641..f66dc5fb161916c4 100755
--- a/src/configure
+++ b/src/configure
@@ -5232,6 +5232,7 @@ else
 		                       res1 = (i == (int)arg);
                              else
 		                       res2 = (i == (int)arg);
+		             return NULL;
                            }
 	                       __thread int i;
                            int main () {
@@ -5360,7 +5361,7 @@ else
                                exit(1);
                              }
                              done = 1;
-	                         return j;
+			     return (void *) j;
                            }
 
                            int main( int argc, char ** argv ) {
diff --git a/src/configure.in b/src/configure.in
index cb0a4b59bbed1256..37ff2a98ef85fcfe 100644
--- a/src/configure.in
+++ b/src/configure.in
@@ -714,6 +714,7 @@ AC_ARG_WITH(tls,
 		                       res1 = (i == (int)arg);
                              else
 		                       res2 = (i == (int)arg);
+		             return NULL;
                            }
 	                       __thread int i;
                            int main () {
@@ -805,7 +806,7 @@ AC_ARG_WITH(virtualtimer,
                                exit(1);
                              }        
                              done = 1;
-	                         return j;
+			     return (void *) j;
                            } 
   
                            int main( int argc, char ** argv ) {