Handle polymorphic types for lib_root and include_root in setup.py

Depending on whether these are created by pkg_config or not they might
be a list of directories or just a string with a single directory.

setup.py

@@ -473,11 +473,17 @@ def build_extensions(self):
                 lib_root = include_root = root
 
             if lib_root is not None:
-                for lib_dir in lib_root:
-                    _add_directory(library_dirs, lib_dir)
+                if isinstance(lib_root, str):
+                    _add_directory(library_dirs, lib_root)
+                else:
+                    for lib_dir in lib_root:
+                        _add_directory(library_dirs, lib_dir)
             if include_root is not None:
-                for include_dir in include_root:
-                    _add_directory(include_dirs, include_dir)
+                if isinstance(include_root, str):
+                    _add_directory(include_dirs, include_root)
+                else:
+                    for include_dir in include_root:
+                        _add_directory(include_dirs, include_dir)
 
         # respect CFLAGS/CPPFLAGS/LDFLAGS
         for k in ("CFLAGS", "CPPFLAGS", "LDFLAGS"):