waf complains about missing features in the builder, so let's add some.

For the sake of completeness, here's an example error message:

Traceback (most recent call last):
  File "/home/adi/jack2/.waf-1.6.11-30618c54883417962c38f5d395f83584/waflib/Runner.py", line 162, in start
    st=tsk.runnable_status()
  File "/home/adi/jack2/.waf-1.6.11-30618c54883417962c38f5d395f83584/waflib/Task.py", line 285, in runnable_status
    new_sig=self.signature()
  File "/home/adi/jack2/.waf-1.6.11-30618c54883417962c38f5d395f83584/waflib/Task.py", line 274, in signature
    self.sig_implicit_deps()
  File "/home/adi/jack2/.waf-1.6.11-30618c54883417962c38f5d395f83584/waflib/Task.py", line 372, in sig_implicit_deps
    (nodes,names)=self.scan()
  File "/home/adi/jack2/.waf-1.6.11-30618c54883417962c38f5d395f83584/waflib/Tools/c_preproc.py", line 594, in scan
    raise Errors.WafError('%r is missing a feature such as "c", "cxx" or "includes": '%task.generator)
WafError: bld(uselib_local='clientlib', posted=True, features=[], idx=28, uselib='RT SNDFILE', meths=['process_rule', 'process_source'], prec=defaultdict(<type 'list'>, {}), includes=['../linux', '../posix', '../common/jack', '../common'], source=[/home/adi/jack2/example-clients/capture_client.c], mappings={}, path=/home/adi/jack2/example-clients, _name='jack_rec', target='jack_rec') in /home/adi/jack2/example-clients is missing a feature such as "c", "cxx" or "includes":