dk2nugenie won't build against system libxml2

[vhewes]

i've been seeing issues with building dk2nugenie, which appear to stem from this line in the recipe:

nusofthep-spack-recipes/packages/dk2nugenie/package.py

Line 56 in 6fbc5a0

self.define("LIBXML2_INC", self.spec["libxml2"].prefix.include),

if spack uses the system's libxml2 package, the headers are located in /usr/include/libxml2/libxml, which means that the include directory needs an additional subdirectory, like

self.spec["libxml2"].prefix.include.libxml2

i'm not sure whether spack provides a means to automatically detect the include paths in a better way, but it would be good to have a recipe that supports both system and spack-installed versions of libxml2, if possible. thank you!