configure: fix empty prefix

autoconf-based configure scripts (which this one is modeled after) support to pass an empty prefix via --prefix=. this is required for systems that have binaries directly in /bin rather than /usr/bin etc (one example is sabotage linux). the fix is easy: rather than testing options.prefix for "if var", which treats an empty string the same as "None", we check for "if var not None". fixes #5197