R: copy hclust code from stats package.

To avoid an R CMD check  warning about calling Fortran code
from another package. I translated the code from Fortran
to C, and put it in a new rinterface_extra.c file,
which will be the place of similar R-specific code
fragments.
R: don't need to do ./bootstrap.sh to create an R package.
R: make sure that the R package does not contain src/Makevars.

Conflicts:
	interfaces/R/Makefile
R: get rid of stats, XML, jpeg, and png dependencies.

stats is loaded automatically, the other three are
only used in an example, so the example now checks
whether they are available.

Conflicts:
	interfaces/R/src/DESCRIPTION