suzerain: #3080 fix CPPFLAGS

Huh.  No idea why modifying AM_CPPFLAGS from within an included
file clobbers the flags.  Anyhow, this stops doing that.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44464 0bd85f0b-c151-43b9-a1e9-27231104551f