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.

docs/doxygen/overviews/envvars.h

@@ -22,7 +22,7 @@ wxWidgets programs.
         (Unix only.)
         Overrides installation prefix. Normally, the prefix
         is hard-coded and is the same as the value passed to @c configure via
-        the @c --prefix switch when compiling the library (typically
+        the @c \--prefix switch when compiling the library (typically
         @c /usr/local or @c /usr). You can set WXPREFIX if you are for example
         distributing a binary version of an application and you don't know in advance
         where it will be installed.}