fix linux builds by keeping the decimal string init in a compiler directive

Author: mredig

Sources/Floating Point Conversion.swift

@@ -49,8 +49,11 @@ extension BigUInt {
         guard integer.sign == .plus else { return nil }
         assert(integer.floatingPointClass == .positiveNormal)
 
-        // keeping around in case the `Decimal._mantissa` property gets deprecated in the future.
-//        let significand = BigUInt("\(integer.significand)")!
+        #if os(Linux)
+        // `Decimal._mantissa` has an internal access level on linux, and it might get
+        // deprecated in the future, so keeping the string implementation around for now.
+        let significand = BigUInt("\(integer.significand)")!
+        #else
         let significand = {
             var start = BigUInt(0)
             for (place, value) in integer.significand.mantissaParts.enumerated() {
@@ -59,6 +62,7 @@ extension BigUInt {
             }
             return start
         }()
+        #endif
         let exponent = BigUInt(10).power(integer.exponent)
 
         self = significand * exponent