Avoid calling exit without declaring it first.  Future compilers will not
accept implicit function declarations.

This is in autoconf-generated code.  Current autoconf does not produce
it anymore, so the issue will go away if upstream ever regenerates the
configure script.

diff -ur hddtemp-0.3-beta15.orig/configure hddtemp-0.3-beta15/configure
--- hddtemp-0.3-beta15.orig/configure	2005-10-17 21:19:28.000000000 +0200
+++ hddtemp-0.3-beta15/configure	2022-12-08 15:59:35.947637944 +0100
@@ -2588,7 +2588,7 @@
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -3332,8 +3332,8 @@
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext