This PR has several modifications for the r package.
- The tk package is always depended on but this pulls in X11, making the
'X' variant non-functional. This PR sets a dependency of tk on '+X'.
- There is a missing dependency on libxmu when '+X' is set.
- The libraries for R wind up in a non-standard location and are thus
left out of the RPATH settings. This PR adds that directory to RPATH.
- The MKL integer interface for gfortran is not in the BLAS libs. This
PR replaces the intel interface with the gfortran interface if needed.
- Use the `LibraryList` `ld_flags` method for blas as that is more in
line with th R Installation and Administration manual.
Note that this PR depends on PR #11956. This PR closes#8642.
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