Enable MEMFS on all platforms so the library isn't looking for /works…

[weech]

…pace/destdir/... files

[weech]

I tested it on x86_64-linux-gnu by linking it to GRIB.jl and running the test suite with it (passing), vs linking GRIB.jl to the artifact in last night's jll (failing due to the absolute path issue). I don't know how well the memory-mapped version works on other platforms though. The other possibility is to have the jll set an environment variable that overrides where the library looks for the definition and sample files.

[giordano]

The other possibility is to have the jll set an environment variable that overrides where the library looks for the definition and sample files.

We currently don't have a good mechanism to do this, even though it's desirable: JuliaPackaging/BinaryBuilder.jl#687. Anyway you can set such environment variable at init time in your package, see the examples in the issue mentioned.

[weech]

I tried it out using the example from GTK.jl and it works to just set the environment variables in GRIB.jl's __init__, so this change isn't necessary. Setting the environment variables is probably required for eventual Windows support anyways, so we'll stick with that solution.