Closed issue by non-Jedi on void-packages repository

https://github.com/void-linux/void-packages/issues/23997

Description:
When running `using Pkg; Pkg.add("Example)` under IJulia, you get the following stacktrace:

```
could not load library "libgit2"
/usr/bin/../lib/libmbedx509.so.0: undefined symbol: mbedtls_mutex_unlock

Stacktrace:
 [1] update_registries(::Pkg.Types.Context, ::Array{Pkg.Types.RegistrySpec,1}; force::Bool) at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/Types.jl:1081
 [2] update_registries at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/Types.jl:1045 [inlined] (repeats 2 times)
 [3] add(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}; preserve::Pkg.Types.PreserveLevel, platform::Pkg.BinaryPlatforms.Linux, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:143
 [4] add(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}) at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:112
 [5] #add#27 at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:109 [inlined]
 [6] add at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:109 [inlined]
 [7] #add#24 at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:107 [inlined]
 [8] add at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:107 [inlined]
 [9] add(::String; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:106
 [10] add(::String) at /builddir/julia-1.4.2/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:106
 [11] top-level scope at In[5]:1
```

The [vendored mbedtls is still at 2.16.0](https://github.com/JuliaLang/julia/blob/release-1.4/deps/Versions.make#L23), so maybe that's the problem? They don't appear to carry any patches for mbedtls, and [don't appear to build it in any strange way](https://github.com/JuliaLang/julia/blob/release-1.4/deps/mbedtls.mk), so that's my best guess at the difference.

Proposed change is to have `USE_SYSTEM_MBEDTLS=0`.