fix quote nesting in split_value_list()

a single quote must not disrupt a double quoted string and vice-versa.

Change-Id: Ibb277bc1c930a7dbe9199ca572507ababbcfae4c
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

qmake/library/qmakeevaluator.cpp

@@ -314,7 +314,8 @@ ProStringList QMakeEvaluator::split_value_list(const QString &vals, const ProFil
         switch (unicode) {
         case '"':
         case '\'':
-            quote = unicode;
+            if (!quote)
+                quote = unicode;
             hadWord = true;
             break;
         case ' ':