Reword, indent, remove temporaries, roll minor

[eddelbuettel]

Nothing major here but we now

• show the (large) indented block for better logic,
• add --clean to R CMD SHLIB (catching just the .o file)
• explicitly remove .cpp, so, .dylib (does that catch all cases?)
• add ChangeLog and NEWS as usual

I wonder if we could ask autoconf for a temp. dir. Probably cleanest.

[kevinushey]

This LGTM and I think obviates the need for my other PR.

[eddelbuettel]

This LGTM and I think obviates the need for my other PR.

I am still wondering if we should something else (other than believing TEMPDIR, which is e.g. empty on my box) as discussed on Slack. And/or make the filename more unique. To be seen...

Also, is there another possible extension besides .so and .dylib we need to worry about?