When using python to check for wx module, call it via the variable

When using python to check for wx module, call it via the variable
earlier set with the path to the python interpreter, rather than
hard-coding 'python'.  Fixes build on systems where python is
installed with a version suffix (e.g. python2.4), such as pkgsrc.

From Berndt Josef Wulf via gnuradio-discuss; change is below the
copyright threshold.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@4017 221aa14e-8319-0410-a670-987f0aec2ac5