notes for "ports compiler"

Currently, USE_GCC without gcc in base will fail:

  "Couldn't find your current GCCVERSION (OSVERSION=${OSVERSION})".

we now have to handle this case.

In devel/clang, set WANT_GCC_VER instead of what is currently USE_GCC.
(Others may change as well, if they are only indicating version instead
of hard dependency.)  USE_GCC will demand gcc be installed, if it is not
already in base.

IWBNI USE_GCC=yes just went and picked up a default version of gcc
per OSVERSION.

USE_CLANG will be introduced to demand clang be installed, if it is not
already in base.

WANT_CLANG_VER will specify a preferred version.

is it necessary to have USE_CC=(clang|gcc) ?

reminder: WANT_ is internal to port framework (not for users to set).