R: compilers portgroup, two fixes

[dstrubbe (David Strubbe)]

Patch to use compilers PortGroup to manage Fortran variants (which adds support for gcc5, gcc6, g95, and dragonegg, and sets gcc5 as default -- passes test phase on my machine). Comments on necessity of clang for the C compiler (at least on Yosemite). Two small fixes: comment in configure.args was ending up in the middle of the configure line since more args are appended to this line, and therefore were probably ignored since they fall after the '#'. The require_active_variants line for tk should not use the path: dependency since tk is set as a port: dependency.