Update MinGW.md

Additional suggestions from #796

Author: bobbrow

Documentation/LanguageServer/MinGW.md

@@ -18,7 +18,8 @@ For developers using MinGW on Windows, we recommend you start with the following
             "defines": [
                 "_DEBUG",
                 "UNICODE",
-                "__GNUC__"
+                "__GNUC__=5",
+                "__cdecl=__attribute__((__cdecl__))"
             ],
             "browse": {
                 "path": [
@@ -34,7 +35,7 @@ For developers using MinGW on Windows, we recommend you start with the following
 }
 ```
 
-The `includePath` above includes the system header paths that gcc uses in version 5.3.0 for C++ projects and matches the output of `gcc -v -E -x c++ -`. The `intelliSenseMode` should be set to **"clang-x64"** to get MinGW projects to work properly with IntelliSense.
+The `includePath` above includes the system header paths that gcc uses in version 5.3.0 for C++ projects and matches the output of `gcc -v -E -x c++ -`. The `intelliSenseMode` should be set to **"clang-x64"** to get MinGW projects to work properly with IntelliSense. The `__GNUC__=#` define should match the major version of the toolchain in your installation (5 in this example).
 
 For C projects, simply remove the c++ lines:
 
@@ -53,7 +54,8 @@ For C projects, simply remove the c++ lines:
             "defines": [
                 "_DEBUG",
                 "UNICODE",
-                "__GNUC__"
+                "__GNUC__=5",
+                "__cdecl=__attribute__((__cdecl__))"
             ],
             "browse": {
                 "path": [