(from Tom Henderson) make valgrind return an error exit code when problems are detected. Closes #449.

wscript

@@ -449,7 +449,7 @@ def get_command_template(*arguments):
     if Options.options.valgrind:
         if Options.options.command_template:
             raise Utils.WafError("Options --command-template and --valgrind are conflicting")
-        cmd = "valgrind --leak-check=full %s"
+        cmd = "valgrind --leak-check=full --error-exitcode=1 %s"
     else:
         cmd = Options.options.command_template or '%s'
     for arg in arguments: