Fix markup on the preprocessor symbols documentation page.

Due to an apparent bug in Doxygen 1.8.2 (the version currently used for the
online documentation generation), using "@c __FOO__" not only renders __FOO__
in fixed width font, but also makes it bold (due to bad interaction with
Markdown support?), so use <tt>__FOO__</tt> instead to work around this.

And in one case, escape underscores explicitly (in __VISUALC7__) as they were
still interpreted, even inside <tt>.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77030 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775