Add /WX switches for MSVC compilers, except clang-msvc

Clang-msvc has problems with around 400 "-Wexpansion-to-defined" errors
in easylogging++.h. No flags seem to be able to disable this error.
Clang-cl is generally in an awkward halfway place between clang and
msvc.

Msvc has problems with C4127 "conditional expression is constant" in the
"qvarlengtharray.h", hence it being disabled with /wd4127.

Neither msvc* compiler has proper support for disabling errors in
specific files. It's either QMAKE magic or waiting for proper support.

advancedSettings.pro

@@ -14,21 +14,14 @@ TARGET = AdvancedSettings
 TEMPLATE = app
 
 *msvc* {
-    #/W4 still has warnings. Including warnings in third party files.
-    #QMAKE_CFLAGS_WARN_ON -= -W3
-    #QMAKE_CXXFLAGS_WARN_ON -= -W3
     #Removing -W3 from both FLAGS is necessary, otherwise compiler will give
     #D9025: overriding '/W4' with '/W3'
-    #QMAKE_CXXFLAGS += /W4
+    QMAKE_CFLAGS_WARN_ON -= -W3
+    QMAKE_CXXFLAGS_WARN_ON -= -W3
+    QMAKE_CXXFLAGS += /W4 /wd4127
     
-    QMAKE_CXXFLAGS += /WX
-    
-    #The MSVC frontend for clang does not understand /external
     !*clang-msvc{ 
-        #external:anglebrackets suppresses warnings made in #includes
-        #made with angle brackets, like #include <iostream>, but will 
-        #still report for includes done with quotes.
-        QMAKE_CXXFLAGS += /experimental:external /external:anglebrackets
+        QMAKE_CXXFLAGS += /WX
     }
 }