llvm-3.3: Fix reset C{XX}FLAGS in configure

llvm commit:

 https://llvm.org/svn/llvm-project/llvm/trunk@174313

.. is broken. Instead of resetting any passed in
C{XX}FLAGS, it tries to set the *value* of those flags
to nothing.

Some other references:

http://llvm.org/bugs/show_bug.cgi?id=16724
https://trac.macports.org/ticket/39761

It may be that as well as this, we should ensure these
flags are empty or undefined before building llvm.

Change-Id: Idcbab66ff9d45392cdd410092aca919fa063c280