Avoid loading libopenfhe_julia_jll when using a custom library

[sloede]

By making the loading of the JLL package conditional on it being actually used, I was able to avoid the error we encountered in hpsc-lab/openfhe-julia#35 (specifically in hpsc-lab/openfhe-julia#35 (comment)):

--- a/src/OpenFHE.jl
+++ b/src/OpenFHE.jl
@@ -1,13 +1,17 @@
 module OpenFHE

 using CxxWrap # need to use everything to avoid `UndefVarError`s
-using Preferences: @load_preference, set_preferences!, delete_preferences!
+using Preferences: @has_preference, @load_preference, set_preferences!, delete_preferences!
 using UUIDs: UUID
-using openfhe_julia_jll: libopenfhe_julia


-# Load library path from preferences and wrap OpenFHE module
-const libopenfhe_julia_path = @load_preference("libopenfhe_julia", libopenfhe_julia)
+# Load library path from preferences or JLL package and wrap OpenFHE module
+if @has_preference("libopenfhe_julia")
+    const libopenfhe_julia_path = @load_preference("libopenfhe_julia")
+else
+    using openfhe_julia_jll: libopenfhe_julia
+    const libopenfhe_julia_path = libopenfhe_julia
+end
 @wrapmodule(() -> libopenfhe_julia_path)

 function __init__()

@ArseniyKholod It would be great if you could try this out and check if it has other downsides - if not, maybe this is useful for the future.

[ArseniyKholod]

Thanks for taking the time for dealing with this problem! I will figure out if this approach has downsides!