R extension dependencies with compiler wrapper

This commit introduces a mechanism to insure that R package dependencies
are built with the Spack compiler wrapper. A copy of Makeconf is made
before `filter_compilers` is called. This is then pointed to by the
R_MAKEVARS_SITE environment variable set up in
`setup_dependent_environment`. With this the normal compilers are used
outside of spack and the spack wrapper compilers are used inside of
spack.

This commit also standardizes on the `join_path` call. It also sets the
commented build command to reflect what is actually used with the newer
string formatting.

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