Avoid using "do { ... } while ( wxFalse )" pseudo-loop.

This loop can't be optimized away by the compiler because wxFalse is an extern
variable which can't be known to be always false. Additionally, this creates
many false positives from Coverity as it assumes that the loop can be executed
more than once.

Define wxSTATEMENT_MACRO_BEGIN/END macros abstracting the exact solution used
and replace wxFalse with "(void)0, 0" for now as this seems to placate MSVC
(which warns about using a bare "0" as a condition) while still allowing the
loop to be completely optimized away.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73101 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/cpp.h

@@ -79,6 +79,28 @@
  */
 #define wxEMPTY_PARAMETER_VALUE /* Fake macro parameter value */
 
+/*
+    Helpers for defining macros that expand into a single statement.
+
+    The standatd solution is to use "do { ... } while (0)" statement but MSVC
+    generates a C4127 "condition expression is constant" warning for it so we
+    use something which is just complicated enough to not be recognized as a
+    constant but still simple enough to be optimized away.
+
+    Another solution would be to use __pragma() to temporarily disable C4127.
+
+    Notice that wxASSERT_ARG_TYPE in wx/strvargarg.h relies on these macros
+    creating some kind of a loop because it uses "break".
+ */
+#ifdef __WATCOMC__
+    #define wxFOR_ONCE(name) for(int name=0; name<1; name++)
+    #define wxSTATEMENT_MACRO_BEGIN wxFOR_ONCE(wxMAKE_UNIQUE_NAME(wxmacro)) {
+    #define wxSTATEMENT_MACRO_END }
+#else
+    #define wxSTATEMENT_MACRO_BEGIN  do {
+    #define wxSTATEMENT_MACRO_END } while ( (void)0, 0 )
+#endif
+
 /*
     Define __WXFUNCTION__ which is like standard __FUNCTION__ but defined as
     NULL for the compilers which don't support the latter.