[libstdc++ PATCH] libstdc++ debug mode (second try)

[John Levon]

I've just fallen foul of trying to use the pedantic mode. The docs
mislead me into just using -D_GLIBCPP_DEBUG_PEDANTIC, whereas you need
that *and* -D_GLIBCPP_DEBUG

Could you clarify that part of the docs ? Even better would be if
_PEDANTIC implied the other define too

thanks
john