Enable GPU pre-compilation

[mfherbst]

Superseeds #1058 based on @abussy's work.

[mfherbst]

@abussy I am really confused by this. Without the check in d5c3d13 the precompilation fails because some methods defined in LibxcCudaExt are not available.

However, this suggests that with this additional check, GPU precompilation will be a avoid operation --- which is also what I find in my experiments (i.e. that on our GPU machine I see no difference with and without this PR).

[abussy]

For the GPU precompilation to work properly, one needs the latest version of Julia LTS and of the JuliaGPU stack. The test on !isnothing(Base.get_extension(Libxc, :LibxcCudaExt)) is trying to catch this exactly. Before some recent changes were made, extensions were not visible at precompilation. I suspect your testing setup is a little old

[mfherbst]

Ok, thanks, I'll try that.

[mfherbst]

Cool, thanks it works. Saves about a factor of 5 in the startup cost, very impressive.