Escape double dashes in Doxygen input to preserve them

By default double dashes are converted to en-dashes in the output (and triple
ones -- to em-dashes), but this is undesirable when double dashes are used not
as a punctuation mark but in command line options or as C++ decrement
operator, so escape them to avoid such conversion in this case.

interface/wx/app.h

@@ -311,7 +311,7 @@ public:
     virtual bool OnCmdLineError(wxCmdLineParser& parser);
 
     /**
-        Called when the help option (@c --help) was specified on the command line.
+        Called when the help option (@c \--help) was specified on the command line.
         The default behaviour is to show the program usage text and abort the program.
 
         Return @true to continue normal execution or @false to return

interface/wx/hashmap.h

@@ -146,7 +146,7 @@
     ++it ( and it++ ) to move to the next element, *it to access the element pointed to,
     it->first ( it->second ) to access the key ( value ) of the element pointed to.
 
-    Hash maps provide forward only iterators, this means that you can't use --it,
+    Hash maps provide forward only iterators, this means that you can't use \--it,
     it + 3, it1 - it2.
 
 

interface/wx/hashset.h

@@ -131,7 +131,7 @@
     An iterator is similar to a pointer, and so you can use the usual pointer
     operations: ++it ( and it++ ) to move to the next element, *it to access the
     element pointed to, *it to access the value of the element pointed to.
-    Hash sets provide forward only iterators, this means that you can't use --it,
+    Hash sets provide forward only iterators, this means that you can't use \--it,
     it + 3, it1 - it2.
 
     @library{wxbase}

interface/wx/log.h

@@ -1282,7 +1282,7 @@ void wxVLogInfo(const char* formatString, va_list argPtr);
     activated by calling wxLog::SetVerbose().
 
     Notice that this is done automatically by wxWidgets, unless the standard
-    command line handling is overridden, if @c --verbose option is specified on
+    command line handling is overridden, if @c \--verbose option is specified on
     the program command line, so using these functions provides a simple way of
     having some diagnostic messages not shown by default but which can be
     easily shown by the user if needed.

interface/wx/textctrl.h

@@ -1613,7 +1613,7 @@ public:
         inheriting wxTextCtrl from @c std::streambuf in which case this class is
         not compiled in.
         You also must have @c wxUSE_STD_IOSTREAM option on (i.e. set to 1) in your
-        @c setup.h to be able to use it. Under Unix, specify @c --enable-std_iostreams
+        @c setup.h to be able to use it. Under Unix, specify @c \--enable-std_iostreams
         switch when running configure for this.
 
     Example of usage: