Port configure script to C99, avoding implicit function declarations.
Future compilers are likely not to accept them by default.

There is no need to upstream this because re-running autotools should
rewrite these file sections to use C99-compatible constructs.

diff --git a/aclocal.m4 b/aclocal.m4
index 38392ca9cfc50528..bc36662d3857dafa 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -756,10 +756,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -773,7 +769,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }]
 EOF
   if AC_TRY_EVAL(ac_link) && test -s conftest${ac_exeext} 2>/dev/null; then
diff --git a/configure b/configure
index 3008b7005e03f440..764248205d537b62 100755
--- a/configure
+++ b/configure
@@ -2932,7 +2932,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -4152,8 +4152,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -8747,10 +8747,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -8764,7 +8760,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -8845,10 +8841,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -8862,7 +8854,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -12451,10 +12443,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -12468,7 +12456,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -12549,10 +12537,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -12566,7 +12550,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -17962,10 +17946,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -17979,7 +17959,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -18060,10 +18040,6 @@ else
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -18077,7 +18053,7 @@ int main ()
       /* dlclose (self); */
     }
 
-    exit (status);
+    return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -19479,8 +19455,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext