Reported here:
https://groups.google.com/forum/#!topic/ns-3-users/KSzjVKX54dE

The error message is:
AirTom:ns-3-dev pecos$ ./waf
Waf: Entering directory `/Users/pecos/Development/workspace/tépè/ns-3-dev/build'
Waf: Leaving directory `/Users/pecos/Development/workspace/tépè/ns-3-dev/build'
Build failed
Traceback (most recent call last):
  File "/Users/pecos/Development/workspace/tépè/ns-3-dev/.waf-1.7.16-9ca17eb492c97b689870b4ff9db75880/waflib/Runner.py", line 162, in start
    st=tsk.runnable_status()
  File "/Users/pecos/Development/workspace/tépè/ns-3-dev/src/wscript", line 504, in runnable_status
    return super(ns3header_task, self).runnable_status()
  File "/Users/pecos/Development/workspace/tépè/ns-3-dev/.waf-1.7.16-9ca17eb492c97b689870b4ff9db75880/waflib/Task.py", line 292, in runnable_status
    key=self.uid()
  File "/Users/pecos/Development/workspace/tépè/ns-3-dev/src/wscript", line 490, in uid
    up(x.abspath().encode())
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 36: ordinal not in range(128)

The workaround is obvious, don't use accents in the folder names. However the error message is not obviously pointing out this.
Waf error messages are outside of our scope, but the error itself is in our own wscript.