ndnSIM

When ns-3 is configured with @--disable-python@, ndnSIM compilation terminates with error:

Waf: Entering directory /Users/user/src/ns-3/build'
[ 584/2095] pyc: src/ndnSIM/PyNDN/Data.py -> build/src/ndnSIM/PyNDN/Data.pyc
/bin/sh: -c: command not found
Waf: Leaving directory/Users/user/src/ns-3/build'
Build failed
-> task in '' failed (exit status 127):
{task 4450737104: pyc Data.py -> Data.pyc}
' -c "import sys, py_compile; py_compile.compile(sys.argv[1], sys.argv[2], sys.argv[3], True)" ../src/ndnSIM/PyNDN/Data.py /Users/user/src/ns-3/build/src/ndnSIM/PyNDN/Data.pyc /PyNDN/Data.py && true '

PyNDN emulation should not be compiled when @--disable-python@ is specified.