[Rd] LDFLAGS defined in R_MAKEVARS_USER file is ignored for R CMD SHLIB on Windows

[Jim Hester]

Example input and output to reproduce this can be found at
https://gist.github.com/jimhester/b7f05f50794c88e44b17.

I tested this attempting to compile the [digest](
http://cran.r-project.org/web/packages/digest/index.html) package, `run.sh`
and `run.bat` were both run in the package source directory on Ubuntu 14.01
and Windows 7 respectively.

In particular while the `CFLAGS` values were properly passed to the
compiler on both Linux and Windows, the `LDFLAGS` value was only passed to
the linker on Linux, which caused the subsequent linking errors on Windows.

Perhaps this is intended behavior, if so is there a different compiler
variable I can use to pass flags to the linker on Windows?

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