Use distutils to find the ghostscript executable?

[vinayak-mehta]

Use distutils.spawn.find_executable instead of the current function as proposed by @ozak in atlanhq/camelot#426.

Also, @fmassa wrote in atlanhq/camelot#282 (comment):

My guess is that there is an issue with ctypes.util.find_library (or is a limitation of its search path), but I didn't dig further to find if / where it is.
Maybe if we were to add the ghostscript location to LD_LIBRARY_PATH ctypes would find it.