Add wxDO_IF() helper macro

This will be used for wxLogXXX() macros reimplementation in the next
commit.

include/wx/cpp.h

@@ -96,6 +96,27 @@
 #define wxSTATEMENT_MACRO_BEGIN  do {
 #define wxSTATEMENT_MACRO_END } while ( (void)0, 0 )
 
+/*
+    Helper for executing the following statement conditionally without using
+    conditional statements.
+
+    This strange macro is needed in the first place to avoid the problems due
+    to nested if/else inside macros. E.g. if some MACRO started with "if", then
+
+        if ( cond )
+            MACRO();
+        else
+            ...
+
+    would be broken because "..." would bind to the wrong "if" inside the macro
+    rather than the visible one. So we use wxDO_IF() inside the macro instead
+    to avoid this problem.
+ */
+#define wxDO_IF_HELPER(loopvar, condition)                                    \
+    for ( bool loopvar = false; !loopvar && condition; loopvar = true )
+
+#define wxDO_IF(condition) wxDO_IF_HELPER(wxMAKE_UNIQUE_NAME(wxdoif), condition)
+
 /*
     Define __WXFUNCTION__ which is like standard __FUNCTION__ but defined as
     NULL for the compilers which don't support the latter.