tries to guess the install directory. You should pass in the value of
argv (the name your program was run as) as a clue to help it out, plus
the name of the environment variable you let the user override your package
install area with (eg. "VIPSHOME").
returns the prefix it discovered, and as a side effect, sets the environment
variable (if it's not set).
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!
works exactly as
but, if vips has not been moved since configure, returns the libdir configure
setting. If vips has been moved, it returns prefix/lib.