Mk/Uses/cran: add "compiles" argument.

[1] Add "compiles" argument to cran to allow R-cran ports need to compile
code to bring in the correct compiler dependencies. Also, ensure
consistency between the compiler selected by the R port and the R-cran ports.

[2] Migrate R-cran ports that manually specify the compiler to
USES=cran,compiles.

PR: 210343
Approved by: wen (1), portmgr (2, blanket)