const char *im_guess_prefix( const char *argv0, const char *env_name )
const char *im_guess_libdir( const char *argv0, const char *env_name )
On success, im_guess_prefix(3) returns the prefix it discovered, and as a side effect, sets the environment variable (if it's not set).
im_guess_prefix(3) is useful for programs in the VIPS distribution and contrib which need to be able to find data files.
Don't free the return string!
im_guess_libdir(3) works exactly as im_guess_prefix(3) but, if vips has not been moved since configure, returns the libdir configure setting. If vips has been moved, it returns prefix/lib.