Check for mdds >= 0.5.0 as sc now needs it...

Otherwise we have problems with paths containing spaces - they don't get
quoted in some cases.

Conflicts:
	scp2/source/extensions/directory_extensions.scp

Here it only gets used for non-Windows and non-Linux. It will current defaults
to using these "internal" headers like we do for xrender and friends.

Configure fails on mdds/mixed_type_matrix.hpp test when testing external
mdds otherwise.