Improve README.maintainer.md to also explain how to build a custom (new) version of a JLL

[fingolfin]

Requested by @fieker

I think this would amount to something roughly like this (??)

UPDATED based on comments by @thofma

git clone https://github.com/JuliaPackaging/Yggdrasil
cd Yggdrasil

# record the base path
BASEPATH=$(pwd)

# ensure building macOS binaries will work (you can omit this if you only
# want to build for Linux)
export BINARYBUILDER_AUTOMATIC_APPLE=true

# change into the directory containing the `build_tarballs.jl` we want to build
cd PATH/TO/PACKAGE

# ensure BinaryBuilder etc. is installed in the right version
# (ideally use the same Julia version as specified in `.ci/Manifest.toml`)
julia --project=$BASEPATH/.ci -e 'using Pkg; Pkg.instantiate()'

# get list of platforms etc.
julia --project=$BASEPATH/.ci build_tarballs.jl --help

# build and deploy the JLL locally. If you omit the comma-separated
# list of PLATFORMS then it will build for *all* platforms
julia --project=$BASEPATH/.ci build_tarballs.jl PLATFORMS --deploy=local

[thofma]

looks good

[thofma]

Ah, no, not quite right. One has to run the build_tarballs.jl from within the P/path_to directory. Because the local patches have a relative path with respect to the build_tarballs.jl. (I always run into this with flint)

And run with BINARYBUILDER_AUTOMATIC_APPLE=true.

[fingolfin]

Updated according to @thofma's helpful comments, thank you

[fingolfin]

Started adding this in PR #667.

But now I thought about it and wonder if we can't also make this (overriding Singular_jll with a custom built variant of Singular) wouldn't be much easier (and faster) if we instead provided instructions and/or a script that follows one of the methods described at https://docs.binarybuilder.org/stable/jll/#Overriding-the-artifacts-in-JLL-packages -- dev_jll() sound pretty nice, and also the use of "preferences".

I mention the possibility for a script just because in order to build Singular in a way that makes it possible to use the result as replacement in the JLL may require to use configure arguments close (or ideally: identical) to what we use in S/Singular/build_tarballs.jl. For that a simple script might be useful...