need help building port with clang

[AN]

FreeBSD FBSD10 10.0-CURRENT FreeBSD 10.0-CURRENT #18 r240078: Mon Sep  3 
17:41:46 EDT 2012     root at FBSD10:/usr/obj/usr/src/sys/MYKERNEL  amd64

clang -v
FreeBSD clang version 3.2 (trunk 162107) 20120817
Target: x86_64-unknown-freebsd10.0

I have been building Firefox-nightly with gcc48 for several months with 
(USE_GCC=4.8+ in makefile).  It stopped working recently (I believe the 
project switched to clang recently and now gcc is not working).

cc -v
Using built-in specs.
Target: amd64-undermydesk-freebsd
Configured with: FreeBSD/amd64 system compiler
Thread model: posix
gcc version 4.2.1 20070831 patched [FreeBSD]

I use the base compiler for world and most ports, I do not want to change 
at this time.  So, my question is how can I tell Firefox-nightly to use 
clang?  Is there an equivalent to USE_GCC=4.8+ for clang that I can put in 
the makefile?  I want to use clang to compile this specific port, and let 
the base compiler build the rest.

Any help is appreciated, thanks in advance.