Create a default "all" target to build all executables
Since Subzero now uses LLVM utilities all over the place, use llvm-config too.

Currently using llvm-config --cxxflags because the Ice* code only uses LLVM
headers. At some point --ldflags --libs will be used too, like it's done for
llvm2ice.
Now that all of Subzero uses llvm-config --cxxflags, kill the custom macro defs.