Fix another implicit function declaration in configure

As mentioned on bug #80171. This one is in libtool.m4, might get
lost on libtool updates.

Author: nikic

build/libtool.m4

@@ -978,10 +978,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -997,7 +993,7 @@ int main ()
   else
     puts (dlerror ());
 
-    exit (status);
+    return (status);
 }]
 EOF
   if AC_TRY_EVAL(ac_link) && test -s conftest${ac_exeext} 2>/dev/null; then