BUG: inconsistent -fPIC causes adios/mxml linkage problem (fix #4605) (#4606)

- Thanks to pointer from @adamjstewart - can avoid this either by
  avoiding an external mxml dependency, or by making certain that they
  both use consistent code generation.

  The adios package uses '-fPIC' for python linkage and this seems to
  require the same for mxml too. If the adios-internal version of mxml
  is used, it has the same flags for both.

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

@@ -48,5 +48,9 @@ def url_for_version(self, version):
             return 'https://github.com/michaelrsweet/mxml/releases/download/release-{0}/mxml-{0}.tar.gz'.format(version)
 
     def configure_args(self):
-        # Default is non-shared, but avoid any future surprises
-        return ['--disable-shared']
+        return [
+            # ADIOS build with -fPIC, so we need it too (avoid linkage issue)
+            'CFLAGS=-fPIC',
+            # Default is non-shared, but avoid any future surprises
+            '--disable-shared',
+        ]