make split_value_list() a bit less sane again

let the backslash escape only quotes (and itself), after all - $$list()
(one of the main users of this function) is commonly used with (windows)
path lists, so letting it escape anything would make a royal mess.

Change-Id: I2dae2e4508903e62c190c1497da68d53978118e7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

qmake/library/qmakeevaluator.cpp

@@ -297,8 +297,13 @@ ProStringList QMakeEvaluator::split_value_list(const QString &vals, const ProFil
             build += QChar(unicode);
             break;
         case '\\':
-            if (x + 1 != vals_len)
+            if (x + 1 != vals_len) {
-                unicode = vals_data[++x].unicode();
+                ushort next = vals_data[++x].unicode();
+                if (next == '\'' || next == '"' || next == '\\')
+                    unicode = next;
+                else
+                    --x;
+            }
             // fallthrough
         default:
             hadWord = true;