llvm3[345]: unconditionally build clang and libcxx
llvm3[345]: simplify installing extras
llvm3[345]: inline variables
llvm3[345]: remove comment that probably isn't relevant anymore
llvm3[345]: combine conditionals
llvm3[345]: simplify caveats
llvm3[345]: remove comments that repeat the code
llvm3[345]: collect 10.6-specific code in one place
llvm33: add 3.3_1 bottle.
llvm34: add 3.4.2 bottle.
llvm35: add 3.5.0 bottle.