leveldb: quote CC and CXX so that they can contain whitespace

For example, CC="ccache gcc" or CXX="ccache g++"
squash me. autotools: quote vars when passing them to leveldb make
squash me. autotools: add ccache support

Speeds builds up a substantial amount, as expected. Local tests show full clean
builds are 25x-40x faster.
squash me. autotools: only test for boost_test if tests are enabled
squash me. gitian linux build needs libboost-test