Fix warning on non-MSVC compilers.

Some Microsoft's specific #pragma were left in the code. They causes the
following errors to be printed:
```
/tmp/antlr4/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp:1351:9:
warning: unknown pragma ignored [-Wunknown-pragmas]
may be unsafe. Consider using _dupenv_s instead.
        ^
/tmp/antlr4/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp:1361:9:
warning: unknown pragma ignored [-Wunknown-pragmas]
```

This patch removes the #pragma macros and uses the matching compiler
definition only on MSVC compiler.

Author: ArthurSonzogni

runtime/Cpp/CMakeLists.txt

@@ -66,7 +66,7 @@ if(WITH_DEMO)
 endif(WITH_DEMO)
 
 if(MSVC_VERSION)
-  set(MY_CXX_WARNING_FLAGS "  /W4")
+  set(MY_CXX_WARNING_FLAGS "  /W4 /wd4996")
 else()
   set(MY_CXX_WARNING_FLAGS "  -Wall -pedantic -W")
 endif()

runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp

@@ -1348,8 +1348,6 @@ Parser* ParserATNSimulator::getParser() {
   return parser;
 }
 
-#pragma warning (disable:4996) // 'getenv': This function or variable may be unsafe. Consider using _dupenv_s instead. 
-
 bool ParserATNSimulator::getLrLoopSetting() {
   char *var = std::getenv("TURN_OFF_LR_LOOP_ENTRY_BRANCH_OPT");
   if (var == nullptr)
@@ -1358,8 +1356,6 @@ bool ParserATNSimulator::getLrLoopSetting() {
   return value == "true" || value == "1";
 }
 
-#pragma warning (default:4996)
-
 void ParserATNSimulator::InitializeInstanceFields() {
   _mode = PredictionMode::LL;
   _startIndex = 0;