Here are some notes for the bsd.compiler.mk first draft.  It is a file
which will be unconditionally included (early) in bsd.port.mk.

It is based in part on the following (incomplete) GSoC work, with
rework by me:

http://p4db.freebsd.org/fileViewer.cgi?FSPC=//depot/projects/soc2010/clangports/ports/Mk/bsd.compiler.mk&REV=3

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The concept of PORTS_CC is introduced for optional use by the user.
It may be set in e.g. /etc/make.conf to e.g. 'gcc', 'clang', or
something else.

  If not set, it defaults to gcc (_not_ whatever is base cc; this
  is kind of the point :-) )

  If set to 'gcc*', that value is extracted and used like USE_GCC is
  currently used.  (See bsd.gcc.mk.)  To be honest, I haven't taken
  that block of code apart to check it.

  If set to 'clang' or something else, CC and CXX are set to follow
  PORTS_CC.  (But see "clang note", below.)

The concept of USE_CC is introduced for optional use in a port
Makefile.  If undefined, CC and CXX are set to whatever PORTS_CC
specifies.  If set to 'base', CC and CXX are set to whatever the
system default is (e.g. current behavior); this is needed for the
ports lang/gcc* and lang/clang*, for example, to avoid chicken-
and-egg.

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I am quite sure that this patch is insufficient.  Comments welcome.

(clang note: there is code in bsd.gcc.mk to be able to depend on a
particular version of gcc or "particular version or later".  This
functionality probably needs to be included for clang as well, but
let's start with this code.)

mcl

Post Scriptum: I am not married to the string "PORTS_CC".  It may be
painted whatever color.