--enable-expensive-checks allows the developer to enable runtime
checking that can greatly increase compile time. Currently it only
turns on _GLIBCXX_DEBUG. Other expensive debugging checks added later
should be controlled by this configure option.
This patch also updates llvm-config with a --cppflags option to inform
llvm-gcc how to build itself so that it is compatible with an llvm that
was built with _GLIBCXX_DEBUG.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37777 91177308-0d34-0410-b5e6-96231b3b80d8