make quoting in the prl writer more robust

this is just a fragment of the original commit and doesn't do anything
in qtcreator. it's there to reduce the diff.

Change-Id: Idd9f64ac608b7e4ed840d5d9925bf741e03d78ab
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
(cherry picked from qtbase/009df82f1ada7c934931efab2950f4bf356c90a6)

src/shared/proparser/qmakebuiltins.cpp

@@ -215,8 +215,8 @@ static QString windowsErrorCode()
 }
 #endif
 
-static QString
-quoteValue(const ProString &val)
+QString
+QMakeEvaluator::quoteValue(const ProString &val)
 {
     QString ret;
     ret.reserve(val.size());

src/shared/proparser/qmakeevaluator.h

@@ -251,6 +251,8 @@ public:
     enum { m_skipLevel = 0 };
 #endif
 
+    static QString quoteValue(const ProString &val);
+
 #ifdef PROEVALUATOR_DEBUG
     void debugMsgInternal(int level, const char *fmt, ...) const;
     void traceMsgInternal(const char *fmt, ...) const;