Fall back on using RTLD_DEFAULT if we can't find flint in Padic

If openSharedLibrary fails but we can still find fmpz_init using
foreignFunction (which wraps dlsym), then we just set flint to be null
and use the new foreignFunction methods that accept null as a first
argument.

Author: d-torrance

M2/Macaulay2/packages/Padic.m2

@@ -56,7 +56,10 @@ endpkg = msg -> (
 if not ForeignFunctions#"private dictionary"#?"foreignFunction"
 then endpkg "foreign function interface is not available"
 
-flint = try openSharedLibrary "flint" else endpkg "flint is not available"
+flint = (
+    try openSharedLibrary "flint"
+    else try foreignFunction("fmpz_init", void, voidstar) then null
+    else endpkg "flint is not available")
 
 export {
     -- methods