Show a helpful error message when SASS_LIBSASS_PATH isn't defined.

Otherwise, you just get an unhelpful error message like this:

    make: option requires an argument -- C
Merge pull request #46 from svnieuw/master

Use getopt_long() instead of getopt_long_only() to parse arguments.
Merge pull request #47 from russelldavis/makefile-error-check

Show a helpful error message when SASS_LIBSASS_PATH isn't defined.