gcc 4.8 compile fix

make ProxyIterator::reference a reference (not a value) in order to truly
satisfy random access iterator requirements, because std algs were trying to
std::swap rvalues

added swap(ProxyIterator &,ProxyIterator &) and (shallow) swap of
SizedProxy.Inner() to go with existing deep swap for SizedProxy; I know there
are other ProxyIterators but the code I'm building doesn't need swap for
those. added a //TODO: comment with a concern I have about SizedProxy's deep
swap (which was only recently added); I (graehl) don't understand KenLM well
enough to judge.

avoid some new gcc 4.8 warnings

made compile_query_only.sh respect CXX env var