[libcalamares] Compatibility-layer for QString::split

- QString::split() api changed in 5.14, in 5.15 generates warnings,
  so introduce a compatibility value.

src/libcalamares/utils/String.h

@@ -33,6 +33,27 @@
 
 #include <QString>
 
+/* Qt 5.14 changed the API to QString::split(), adding new overloads
+ * that take a different enum, then Qt 5.15 deprecated the old ones.
+ * To avoid overly-many warnings related to the API change, introduce
+ * Calamares-specific constants that pull from the correct enum.
+ */
+constexpr static const auto SplitSkipEmptyParts =
+#if QT_VERSION < QT_VERSION_CHECK( 5, 14, 0 )
+    QString::SkipEmptyParts
+#else
+    Qt::SkipEmptyParts
+#endif
+    ;
+
+constexpr static const auto SplitKeepEmptyParts =
+#if QT_VERSION < QT_VERSION_CHECK( 5, 14, 0 )
+    QString::KeepEmptyParts
+#else
+    Qt::KeepEmptyParts
+#endif
+    ;
+
 /**
  * @brief The CalamaresUtils namespace contains utility functions.
  */