specs file method for detecting when preprocessor is being called for C++?

[Christopher Faylor <cgf at redhat dot com>]

I'm probably missing something obvious, but I can't find any method for
setting up a rule which adds additional options to "cpp" when a user is
invoking g++.

I've tried something like:

%{Dcplusplus:blah}

but that doesn't work.

Can anyone tell me if this is possible?  I'd like to be able to do this
with both 2.95.3 and gcc 3.0, if possible.  I don't know if the methods
would be different for each.

My reason for wanting this is that I want to completely separate the
include file parsing for -mcygwin/-mno-cygwin.  In the -mno-cygwin case
the g++ include file search path should include /usr/include/mingw/g++
in the -mcygwin case, it should include /usr/include/g++.

I can't seem to find any way to add these include search paths only when
g++ is invoked.

Any pointers are appreciated.

cgf