[R-SIG-Mac] build R for macOS with binary compatibility with CRAN

[Jonathon Love]

hi,

i've successfully built R for macOS, and it all works well, however i'm 
wanting to achieve compatibility with the prebuilt .tgz CRAN R packages.

the compatibility issue i've been having is that the R produced lacks 
the Resources/lib/libc++.1.dylib. the prebuilt CRAN R packages link to 
this, and can't find it at runtime.

i'm wondering where this file (and a few other files in Resources/lib) 
that ship with the official CRAN R binary come from - because i don't 
think they're from the build process of R.

it looks like the official binary is subject to a few additional build 
steps (copying binaries in, etc.)?

with thanks

jonathon