Use all available CPUs when building

Automatically set Options.options.jobs to the number of CPUs/cores on the
executing machine, unless the environment variable JOBS is set

wscript

@@ -19,6 +19,12 @@ blddir = 'build'
 jobs=1
 if os.environ.has_key('JOBS'):
   jobs = int(os.environ['JOBS'])
+else:
+  try:
+    import multiprocessing
+    jobs = multiprocessing.cpu_count()
+  except:
+    pass
 
 def set_options(opt):
   # the gcc module provides a --debug-level option