sdk: use 7z rather than building xar
qt: remove qt4 build and ugly patches
qt: whoops, don't hard-code here
all: A smattering of fixes.

This will be the last set of changes until after Bitcoin has accepted this work
upstream. The next step is to move the .mk logic directly into the Gitian
descriptors as a stop-gap. After that is done and Upstream can build/run
reliably, a more suitable workflow should be established.

Changed:

- Install the toolchain in native-prefix rather than copying it all over the
  place.
- Install the toolchain headers in native-prefix relative to clang. Now we no
  longer have to include them by hand, since clang looks for them here. This
  mirrors how a distro-provided clang should look, so that change-over will
  be much simpler.
- Use llvm's libLTO.so rather than building it from Apple's possibly-
  incompatible llvmgcc. This means we no longer have to use the llvmgcc sources
  or build llvm from source.
- Use the llvm headers supplied by clang for ld64's lto support. As a quick
  hack, ld64 is given a relative (to the binary) rpath entry in order to find
  libLTO.so without LD_PRELOAD nastiness.
- Use a distro-provided libuuid rather than building from source
- Use 7z for extracting rather than building xar.
- Remove unnecessary package/source dependencies