Allow preventing unwanted environment variable expansion in Markdown ... blocks

[vadz]

Currently writing something like this:

To use the library, you need to add `-I$(WXWIN)/include` to the compiler options.

results in -I/include in the output because the environment variable gets expanded when Doxygen is run. While I realize that this is done intentionally and it may be useful to do it in other situations, it's definitely not wanted here and I'm not sure if it's ever really desirable to do it inside Markdown backticks.

So I think Doxygen shouldn't expand environment variables inside backticks by default or, if you think it should, should at least provide some way of disabling it.

Currently the least bad workaround I have is to use

`-I$``(WXWIN)/include`

which is just ugly.

Reference to (old) stack overflow question: https://stackoverflow.com/questions/18499009/doxygen-expands-environment-variable-inside-mainpage

[albert-github]

An less ugly approach might be to escape the $ sign and use the ´\c´:

\c \$(VAR)/dir

[vadz]

Thanks, using \c is indeed better than using <code>, but still looks rather out of place in Markdown files -- notably it's not highlighted as code in the editor by default.