R-1.2.3: a small suggestion (PR#961)

[This is not a bug report, just a suggestion.]

With complex packages, it is often useful to maintain multiple
versions installed, for bug checking, and performance comparisons, and
for developers, for a development history (e.g., I have about 30
versions of ghostscript, whose development I contribute to, and I have
on several occasions run tests files on with each them to find out
when a change in behavior was introduced).

The common way to do this is to suffix a version number to the
executable and any file tree(s) that it requires.

The R-1.2.3 release does not currently do this, but it looks like only
a couple of lines of changes are needed to accomplish the job, and
I've done so locally manually.

(1) In the R script, change

	R_HOME_DIR=/usr/local/lib/R

    to

	R_HOME_DIR=/usr/local/lib/R-1.2.3

    and name the tree that way during installation.

(2) At installation time, first install the executable as
    /usr/local/bin/R-1.2.3, then remove /usr/local/bin/R,
    then do "ln -s R-1.2.3 /usr/local/bin/R" or
    "ln R-1.2.3 /usr/local/bin/R", in that order.  This
    ensures that any existing version of R remains on the system,
    accessible as R-x.y.z.

[Obviously, in the above, "/usr/local" is really "$(prefix)".]

I suggest that the next round of R distribution consider incorporating
this change.