Allow specifying custom location for MKL

[alyst]

Allows specifying custom paths that would be used for MKL injection into sysimg.jl.
The paths default to the current ["@v#.#", "@"], but could be explicitly specified when calling enable_mkl_startup(), or via the JULIA_MKL_LOAD_PATH environment variable (so that it would be picked up when building sysimg upon Pkg.add(MKL)).

The motivation
Now that building Julia in ArchLinux is broken (due to http-parser/libgit2 incompatibilities, see JuliaLang/julia#34785), this PR allows using the official Julia binaries to build MKL-enabled ArchLinux package.
I install MKL into /usr/share/julia/site using this AUR script snippet:

...

build() {
  # rebuild sysimage with MKL support
  SITE_DEPOT_PATH=${srcdir}/julia-${pkgver}/share/julia/site
  rm -rf ${SITE_DEPOT_PATH}
  mkdir -p ${SITE_DEPOT_PATH}
  JULIA_LOAD_PATH="${SITE_DEPOT_PATH}:@stdlib" \
  JULIA_DEPOT_PATH=${SITE_DEPOT_PATH} \
  JULIA_MKL_LOAD_PATH="${SITE_DEPOT_PATH}" \
  ${srcdir}/julia-${pkgver}/bin/julia \
    -H ${srcdir}/julia-${pkgver}/bin \
    --startup-file=no \
    --project=${SITE_DEPOT_PATH} \
    ${srcdir}/build_sysimg_with_mkl.jl
}

package() {
  mkdir -p ${pkgdir}/usr/share/licenses/julia
  cp -r julia-${pkgver}/{bin,etc,include,lib,share} ${pkgdir}/usr/
  rm -rf ${pkgdir}/usr/share/julia/site/{artifacts,clones,logs,registries}
  sed "s!${srcdir}/julia-${pkgver}!/usr!" -i ${pkgdir}/usr/share/julia/base/sysimg.jl 
  install -Dm644 julia-${pkgver}/LICENSE.md \
      ${pkgdir}/usr/share/licenses/julia/LICENSE.md
}

build_sysimg_with_mkl.jl is just:

using Pkg
Pkg.add(PackageSpec(url="https://github.com/alyst/MKL.jl", rev="custom_pkg_location"))

[alyst]

Actually, it would be nicer to use JULIA_MKL_LOAD_PATH="@stdlib/../../site" -- no need to fix sysimg.jl afterwards (though I don't know whether that is relevant at all).
But at the moment Base.load_path_expand() treats anything after "@" that is not literally @stdlib as a name of some environment. Would it make sense fixing Base.load_path_expand()?

[KristofferC]

I still don't really understand why this is needed. Could you elaborate on what is special with ArchLinux that requires this?

[alyst]

I still don't really understand why this is needed. Could you elaborate on what is special with ArchLinux that requires this?

Until recently, the official ArchLinux Julia package was working + I was able to build AUR package with MKL support (because e.g. TensorDecompositions.jl is 10x faster with it).
ATM the ArchLinux-official package and AUR ones are broken and, IIUC, the recommendation from Julia devs is to install JuliaLang-official Julia binaries in the home folder and patch users's sysimg.jl to use MKL.jl from user's depot. BTW, is that right?
I don't want to do it this way, because I would like to keep the version of Julia (and few other software packages like R) on my laptop, on the server and on the Docker/Charliecloud image in sync, and I would like to have a simple, reliable and automated way of doing that.
So far I was pretty happy with ArchLinux and its AUR system for managing custom software I use, so I would like to keep using it, instead of inventing some other approaches.

What I want is that Julia is installed into /usr, as well as MKL.jl (e.g. into share/julia/site), and this is done by the OS (ArchLinux) package manager.
Above I've posted an extract from AUR PKGBUILD that automates this process, so in the end I have a proper ArchLinux package with official Julia binaries and MKL support out-of-the-box.
(Well, now I realize that it's not over yet, and I have to solve the problem with artifacts installed somewhere else)

I hope now it's a bit more clear.

The alternative would be to fix the http-parser/libgit2 issue, so that it would be possible to build Julia (with MKL enabled at compile time) on ArchLinux with its current http-parser-2.9.3 package.

[aminya]

I would like to have a simple, reliable and automated way of doing that.

I recommend other Linux distributions (e.g. Ubuntu, Mint, Fedora, etc.) then.

[ViralBShah]

Also related: #84