always use single quotes in shellQuoteUnix()

double quotes cause mingw32-make to switch from direct execution to going
through the shell, so avoid them.

Change-Id: I05b71a050e425a1b327f747fab01755ff528ba0b
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

qmake/library/ioutils.cpp

@@ -119,7 +119,7 @@ QString IoUtils::shellQuoteUnix(const QString &arg)
     }; // 0-32 \'"$`<>|;&(){}*?#!~[]
 
     if (!arg.length())
-        return QString::fromLatin1("\"\"");
+        return QString::fromLatin1("''");
 
     QString ret(arg);
     if (hasSpecialChars(ret, iqm)) {