Makefile.in, configure.in: cppflags

* Makefile.in, configure.in (cppflags): allow setting cppflags from
  the confiugre command line.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44255 b2dd03c8-39d4-4d8f-98ff-823fe69b080e