qmake: correct the backslash from string. use '/' for qmake

qmake knows how to deal with '/'. '\' is only used for win command.
if '\' appears in qmake file, qmake will give a warning

QtAV.pro

@@ -12,6 +12,7 @@ OTHER_FILES += templates/vo.h templates/vo.cpp templates/COPYRIGHT.h templates/m
 
 #cache mkspecs. compare mkspec with cached one. if not equal, remove old cache to run new compile tests
 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
 } else {
@@ -21,7 +22,9 @@ mkspecs_build = $$[QMAKE_MKSPECS]
 ##TODO: BUILD_DIR=>BUILD_ROOT
 #if not empty, it means the parent project may already set it
 isEmpty(out_dir):out_dir = $$OUT_PWD
+out_dir ~= s,\\\\,/,g #avoid warning for '\'. qmake knows how to deal with '/'
 isEmpty(SOURCE_ROOT):SOURCE_ROOT = $$PWD
+SOURCE_ROOT ~= s,\\\\,/,g #avoid warning for '\'. qmake knows how to deal with '/'
 isEmpty(BUILD_DIR):BUILD_DIR=$$out_dir
 message("BUILD_DIR=$$BUILD_DIR")
 

common.pri

@@ -196,10 +196,10 @@ defineReplace(clean_path) {
     return($$join(out, /, $$pfx))
 }
 
-#make sure BUILD_DIR is already defined. otherwise return the input path
+#make sure BUILD_DIR and SOURCE_ROOT is already defined. otherwise return the input path
 #only operate on string, seperator is always "/"
 defineReplace(shadowed) {
-    isEmpty(SOURCE_ROOT):return($$1)
+    isEmpty(SOURCE_ROOT)|isEmpty(BUILD_DIR):return($$1)
     1 ~= s,$$SOURCE_ROOT,,g
     shadow_dir = $$BUILD_DIR/$$1
     shadow_dir ~= s,//,/,g
@@ -219,9 +219,9 @@ defineReplace(shell_quote) {
 
 defineReplace(system_path) {
     win32 {
-        1 ~= s,/,\\,g
+        1 ~= s,/,\\,g #qmake \\=>put \\=>real \?
     } else {
-        1 ~= s,\\,/,g
+        1 ~= s,\\\\,/,g  ##why is \\\\. real \=>we read \\=>qmake \\\\?
     }
     return($$1)
 }

configure.prf

@@ -31,6 +31,7 @@ defineTest(cache) {
         } else:isEqual(2, sub) {
             varstr = "$$1 -= $$srcval"
         } else:isEqual(2, set) {
+            ##TODO: erase existing VAR in cache
             varstr = "$$1 = $$srcval"
         } else {
             error("cache(): invalid flag $$2.")
@@ -39,7 +40,7 @@ defineTest(cache) {
     #log("varstr: $$varstr")
 ##TODO: remove existing lines contain $$srcvar
     #because write_file() will write 1 line for each value(seperated by space), so the value must be closed with "", then it's 1 value, not list
-#erase the existing value
+#erase the existing var and value pair
     win32 {#:isEmpty(QMAKE_SH) { #windows sucks. can not access the cache
 
     } else {