Recognize MSVC 14 a.k.a. Visual Studio 2015 compiler.

Interpret internal compiler version value 1900 as VC14. Notice that this
required adjusting the computation of the internal version from the
user-visible one because VC13 was skipped (hopefully no black cats crossed
paths with the manager responsible for this decision).

See #16854.

include/msvc/wx/setup.h

@@ -63,6 +63,8 @@
         #define wxCOMPILER_PREFIX vc110
     #elif _MSC_VER == 1800
         #define wxCOMPILER_PREFIX vc120
+    #elif _MSC_VER == 1900
+        #define wxCOMPILER_PREFIX vc140
     #else
         #error "Unknown MSVC compiler version, please report to wx-dev."
     #endif