add force-realclean facility to Cactus build system

[Roland Haas]

there is currently no way to force a clean rebuild when low level files within Cactus change. This happened twice recently (once to introduce cctk_ash and once to split cctk.h into separate files).

In the both case, failing to recompile causes very strange parameter related errors at runtime.

Keyword:

[Erik Schnetter]

Note that Simfactory already offers such a mechanism. This mechanism is also used when there are incompatible changes to option lists, e.g. when switching compilers or major compiler versions.