Update GDAL to find libxml2 pkg-config.

component/gdal/buildout.cfg

@@ -41,8 +41,7 @@ environment =
   PATH=${xz-utils:location}/bin:%(PATH)s
   CPPFLAGS=-I${pcre:location}/include
   LDFLAGS=-L${pcre:location}/lib -Wl,-rpath=${buildout:parts-directory}/${:_buildout_section_name_}/lib -Wl,-rpath=${proj:location}/lib -Wl,-rpath=${curl:location}/lib -Wl,-rpath=${geos:location}/lib -Wl,-rpath=${giflib:location}/lib -Wl,-rpath=${openjpeg:location}/lib -Wl,-rpath=${jbigkit:location}/lib -Wl,-rpath=${libexpat:location}/lib -Wl,-rpath=${libjpeg:location}/lib -Wl,-rpath=${libpng:location}/lib -Wl,-rpath=${libtiff:location}/lib -Wl,-rpath=${libxml2:location}/lib -Wl,-rpath=${openssl:location}/lib -Wl,-rpath=${pcre:location}/lib -Wl,-rpath=${sqlite3:location}/lib -Wl,-rpath=${zlib:location}/lib
-  LIBXML2_CFLAGS=-I${libxml2:location}/include/libxml2
-  LIBXML2_LIBS=-lxml2 -L${libxml2:location}/lib
+  PKG_CONFIG_PATH=${libxml2:location}/lib/pkgconfig
 
 [gdal-python]
 recipe = zc.recipe.egg:custom