qmake: correct Qt version detection. do not empty cache if recheck

QtAV.pro

@@ -15,12 +15,13 @@ OTHER_FILES += \
 #OTHER_FILES += config.test/mktest.sh
 
 #cache mkspecs. compare mkspec with cached one. if not equal, remove old cache to run new compile tests
+#TODO: Qt5 does not have QMAKE_MKSPECS, use QMAKE_SPEC, QMAKE_XSPEC
 mkspecs_build = $$[QMAKE_MKSPECS]
 mkspecs_build ~= s,\\\\,/,g #avoid warning for '\'. qmake knows how to deal with '/'
 !isEmpty(mkspecs_cached) {
-    !isEqual(mkspecs_cached, $$mkspecs_build):new_compile_tests = 1
+    !isEqual(mkspecs_cached, $$mkspecs_build):CONFIG += recheck
 } else {
-    new_compile_tests = 1
+    CONFIG += recheck
 }
 
 ##TODO: BUILD_DIR=>BUILD_ROOT
@@ -32,8 +33,8 @@ SOURCE_ROOT ~= s,\\\\,/,g #avoid warning for '\'. qmake knows how to deal with '
 isEmpty(BUILD_DIR):BUILD_DIR=$$out_dir
 message("BUILD_DIR=$$BUILD_DIR")
 
-greaterThan(QT_MAJOR_VERSION, 5) {
-    !isEmpty(new_compile_tests):write_file($$BUILD_DIR/.qmake.cache)
+greaterThan(QT_MAJOR_VERSION, 4) {
+    #recheck:write_file($$BUILD_DIR/.qmake.cache) #FIXME: empty_file result in no qtCompileTest result in cache
     load(configure)
 } else {
     _QMAKE_CACHE_QT4_ = $$_QMAKE_CACHE_
@@ -44,7 +45,7 @@ greaterThan(QT_MAJOR_VERSION, 5) {
     }
     message("_QMAKE_CACHE_QT4_: $$_QMAKE_CACHE_QT4_")
     include(common.pri)
-    !isEmpty(new_compile_tests):write_file($$BUILD_DIR/.qmake.cache)
+    #recheck:write_file($$BUILD_DIR/.qmake.cache) #FIXME: empty_file result in no qtCompileTest result in cache
     #use the following lines when building as a sub-project, write cache to this project src dir.
     #if build this project alone and do not have sub-project depends on this lib, those lines are not necessary
     ####ASSUME compile tests and .qmake.cache is in project out root dir