Improve cpt a few different ways

[jeaye]

1. Add support for different cling/clang/llvm repos
2. Add support for different branches for cling/clang/llvm
3. Add support for allowing "dirty" builds, which is necessary for anyone trying to use cpt to develop cling without losing changes with each build
4. Add support for -j flag to specify the number of cores to use when building; this is essential for anyone with a small tmpfs and a lot of cores, since I was filling up /tmp before each core could finish. Note that this is part of #406 and that sub task can be marked complete after merging this.

With these changes, it's now possible to do something like this:

$ ./cpt.py -j 4 \
           --create-dev-env=Debug \
           --with-workdir=./cling-build/ \
           --allow-dirty \
           --with-llvm-url="https://github.com/jeaye/llvm" \
           --with-llvm-branch="cling-patches-llvm13" \
           --with-clang-url="https://github.com/vgvassilev/clang" \
           --with-clang-branch="cling-patches-llvm13" \
           --with-cling-url="https://github.com/vgvassilev/cling" \
           --with-cling-branch="cling-patches-llvm13"

[jeaye]

Checking in on the interest in this. :slightly_smiling_face:

[vgvassilev]

@jeaye, can you bring that to https://github.com/root-project/root (interpreter/cling) this is where the development happens generally :)

[jeaye]

Ah, will do. :+1: