[Rivet] rivet-buildplugin

[David Grellscheid]

Hi Andy,

 > and also uses the user env values of CXXFLAGS and CPPFLAGS in the
 > build

In a discussion Hendrik and I had earlier, we came to the conclusion 
that in the specific case of rivet-buildplugin it would be better for 
consistency to unquestioningly take the values that Rivet was configured 
with, and to ignore user variables which can easily produce something 
incompatible.

   David