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

include/wx/compiler.h

@@ -49,7 +49,14 @@
 #   define __VISUALC11__
 #elif __VISUALC__ < 1900
 #   define __VISUALC12__
+#elif __VISUALC__ < 2000
+    /* There is no __VISUALC13__! */
+#   define __VISUALC14__
 #else
+    /*
+        Don't forget to update include/msvc/wx/setup.h as well when adding
+        support for a newer MSVC version here.
+     */
 #   pragma message("Please update wx/compiler.h to recognize this VC++ version")
 #endif
 
@@ -89,7 +96,13 @@
 #   define wxVISUALC_VERSION(major) 0
 #   define wxCHECK_VISUALC_VERSION(major) 0
 #else
-#   define wxVISUALC_VERSION(major) ( (6 + major) * 100 )
+    /*
+        Things used to be simple with the _MSC_VER value and the version number
+        increasing in lock step, but _MSC_VER value of 1900 is VC14 and not the
+        non existing (presumably for the superstitious reasons) VC13, so we now
+        need to account for this with an extra offset.
+     */
+#   define wxVISUALC_VERSION(major) ( (6 + (major >= 14 ? 1 : 0) + major) * 100 )
 #   define wxCHECK_VISUALC_VERSION(major) ( __VISUALC__ >= wxVISUALC_VERSION(major) )
 #endif