Dia requires libxml2. (#3976)

* Dia requires libxml2.

* Clean up dependencies for Dia (and add X11 deps).

+ Remove dependencies on cairo and libpng.  The will be satisfied via gtkplus.
+ Add dependencies on X11 libraries: libsm, libuuid, libxinerama, libxrender.
+ From a dependency diagram, it doesn't appear that we need libxml2 since this
  dependency should be come in through cairo (via gtkplus).  However, Dia will
  not build without it.

var/spack/repos/builtin/packages/dia/package.py

@@ -34,15 +34,18 @@ class Dia(Package):
 
     depends_on('intltool', type='build')
     depends_on('gtkplus@2.6.0:')
-    depends_on('cairo')
-    depends_on('libpng')
     depends_on('libxslt')
     depends_on('python')
     depends_on('swig')
+    depends_on('libsm')
+    depends_on('libuuid')
+    depends_on('libxinerama')
+    depends_on('libxrender')
+    depends_on('libxml2')
 
     # TODO: Optional dependencies, not yet supported by Spack
     # depends_on('libart')
-    # depends_on('py-gtk', type=('build', 'run'))
+    # depends_on('py-pygtk', type=('build', 'run'))
 
     def url_for_version(self, version):
         """Handle Dia's version-based custom URLs."""