Refactored the Boost Build execnt.c module's maxline() function to cache its result instead of calling the GetVersionEx() Windows API on every call.

[SVN r78989]

v2/engine/execnt.c

@@ -599,7 +599,7 @@ static void free_argv( char const * * args )
  *     http://support.microsoft.com/default.aspx?scid=kb;en-us;830473
  */
 
-int maxline()
+static int raw_maxline()
 {
     OSVERSIONINFO os_info;
     os_info.dwOSVersionInfoSize = sizeof( os_info );
@@ -610,6 +610,13 @@ int maxline()
     return 996;                                     /* NT 3.5.1 */
 }
 
+int maxline()
+{
+    static result;
+    if ( !result ) result = raw_maxline();
+    return result;
+}
+
 
 /*
  * Convert a command string into arguments as used by Unix spawnvp() API. The