macros in .md files

[Joern Rennecke]

> The facility already exists for a target to have its MD file passed
> through CPP.    See md_cppflags in configure.in.
> 
> I forget which port used this (apparently none do now), but the
> facility is there.

Indeed, no port in cvs uses it.  It is also not documented.  If this
is supposed to be supported, I suppose it should be documented in md.texi.

I have a few concerns about using cpp, though.

First, can we assume that cpp is available without restricting portability?

Second, won't cpp's concept of C preprocessor tokens cause trouble when
applied to the md language?