Add defined() before #if SYM > val evaluations

- Change "#if SYM > x" to "#if defined(SYM) && SYM > x" as a pattern. In
  StringUtils.h this follows the pattern used earlier in the file.

runtime/Cpp/runtime/src/antlr4-common.h

@@ -77,7 +77,7 @@
 
   class ANTLR4CPP_PUBLIC std::exception; // Needed for VS 2015.
 
-#elif __APPLE__
+#elif defined(__APPLE__)
   typedef std::u32string UTF32String;
 
   #define GUID_CFUUID

runtime/Cpp/runtime/src/support/StringUtils.h

@@ -24,7 +24,7 @@ namespace antlrcpp {
     // Don't make the converter static or we have to serialize access to it.
     UTF32Converter converter;
 
-    #if _MSC_VER >= 1900 && _MSC_VER < 2000
+    #if defined(_MSC_VER) && _MSC_VER >= 1900 && _MSC_VER < 2000
       auto p = reinterpret_cast<const int32_t *>(data.data());
       return converter.to_bytes(p, p + data.size());
     #else
@@ -36,7 +36,7 @@ namespace antlrcpp {
   {
     UTF32Converter converter;
 
-    #if _MSC_VER >= 1900 && _MSC_VER < 2000
+    #if defined(_MSC_VER) && _MSC_VER >= 1900 && _MSC_VER < 2000
       auto r = converter.from_bytes(first, last);
       i32string s = reinterpret_cast<const int32_t *>(r.data());
     #else