swig/python: do not fail if requirements are not met
swig/python: do not fail if requirements are not met

in order to avoid a fatal error if qi_swig_wrap_python
requirements are not met, typical user code is

include(qibuild/swig/python)
qi_add_optional_package(python)
if (WITH_PYTHON)
  qi_swig_wrap_python(mymodule...)
endif()

This pattern relies on the user knowing the implemention
details of qi_swig_wrap_python, and will be obsolete
due to the ongoing work to support python3, which will require
the FindPython2/FindPython3 modules and a very recent cmake
version.

With this patch, the user code can be changed to:

include(qibuild/swig/python)
if (COMMAND qi_swig_wrap_python)
  qi_swig_wrap_python(mymodule...)
endif()

And will remain valid when the qi_swig_wrap_python
requirements are updated.

aldebaran/qibuild,aldebaran/qibuild,aldebaran/qibuild,aldebaran/qibuild