Makefile clean-up (LLDB tarball, and unused variable removal)

This PR changes the way how LLDB is downloaded, replacing the git-based checkout to ordinary tarball fetching just as is done for other LLVM-related tools (clang, compiler-rt). This because in case of LLVM 3.5, the `release_35` branch does not exist, and in order to get it to compile I had to switch to the official 3.5 release tarball of LLDB.
This required adding `python-config` to the `python2` hack, because up to LLDB 3.5 there seems to be a hard-coded dependency on the `python-config` binary, expecting a Python 2 binary but on my (Arch) system resulting in the Python 3 version.

Additionally, I removed the unused `LIBCXX_OPTIONS` variable, as discussed on commit faf6990. I originally wanted to make it conform to how `LIBGIT2_OPTS` is incrementally constructed, but lacking a Windows system I wouldn't be able to test this.