add QMakeOption::do_cache

... because qmake has it.

Change-Id: Idc2c9f6a041ff1d60d95e8a2ad400f7d41f3d2d2
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/shared/proparser/qmakeglobals.cpp

@@ -93,6 +93,8 @@ QMakeGlobals::QMakeGlobals()
 {
     initStatics();
 
+    do_cache = true;
+
 #ifdef Q_OS_WIN
     dirlist_sep = QLatin1Char(';');
     dir_sep = QLatin1Char('\\');