Ensure $(DESTDIR)$(mono_libdir)/mono/3.5 exists before installing to it

Prior to 38348361dadd5d9e8a1721ecd3ea31b01952d1e7, the mono/X.X folders were created when "gacutil /i foo.dll /package X.X" was called. However, the above commit caused a call to $(INSTALL_DATA) to be executed *before* gacutil, which caused a copy of Microsoft.Common.targets to be installed as a file named mono/3.5 instead of into the mono/3.5 folder, on fresh installs (not a problem installing into an existing prefix).

This commit forces creation of the folders which could potentially not have been created yet by gacutil.