libreoffice ignores defined CC variables and friends

[Da Rock]

In an attempt to build libreoffice I'm trying to build 
libreoffice-3.4.5.2 with clang, but it is ignoring every attempt I have 
made to set env variables, configure args.

I have used -DCC=clang -DCXX=clang++ -DCPP=clang-cpp, I edited the 
Makefile to check these and set them in CONFIGURE_ARGS or CONFIGURE_ENV. 
It is not listening and is determined to use gcc :/

Am I missing something fundamental in my understanding of how things 
work here?

Cheers