pkg-config

[Paul Ramsey]

I am re-working the GEOS detection in the configure script a bit, and
I have the following logic:

if (geos-config provided)
  use geos-config
else
  if (pkg-config found)
    use pkg-config
  elseif (geos-config on path)
    use geos-config
  else
    error cannot find geos

Is that the right order? I noticed that libxml2, just prior to geos in
the configure script, prefers xml2-config to pkg-config if it exists.

P