If we can't find the protobuf headers with default CXXFLAGS and
CPPFLAGS, add -I/usr/local/include to them and try again so that this
is more likely to work out of the box.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@884 edb9625f-4e0d-4859-8d74-9fd3b1da38cb