Change constants used in 'ToBool' to support MSVC

The tokens `TRUE` and `FALSE` are defined in current versions of MSVC, breaking compilation of tinyxml2.cpp in MSVC due to the use of these tokens as variable names in the function `XMLUtil::ToBool`.

Simply changing the variable names to something else resolves the problem.
diff --git a/tinyxml2.cpp b/tinyxml2.cpp
index 6b79917..a54d0b9 100755
--- a/tinyxml2.cpp
+++ b/tinyxml2.cpp
@@ -617,17 +617,17 @@
         *value = (ival==0) ? false : true;

         return true;

     }

-    static const char* TRUE[] = { "true", "True", "TRUE", 0 };

-    static const char* FALSE[] = { "false", "False", "FALSE", 0 };

+    static const char* TRUE_VALS[] = { "true", "True", "TRUE", 0 };

+    static const char* FALSE_VALS[] = { "false", "False", "FALSE", 0 };

 

-    for (int i = 0; TRUE[i]; ++i) {

-        if (StringEqual(str, TRUE[i])) {

+    for (int i = 0; TRUE_VALS[i]; ++i) {

+        if (StringEqual(str, TRUE_VALS[i])) {

             *value = true;

             return true;

         }

     }

-    for (int i = 0; FALSE[i]; ++i) {

-        if (StringEqual(str, FALSE[i])) {

+    for (int i = 0; FALSE_VALS[i]; ++i) {

+        if (StringEqual(str, FALSE_VALS[i])) {

             *value = false;

             return true;

         }