The flags added in toolkit/locales/Makefile.in turn out not to be actually
used, so just remove that.
The remaining uses of XULPPFLAGS are to set debug flags depending on whether
MOZ_DEBUG is set or not. Just set a dedicated variable with the right value
from configure.
This hack has actually not been actively used since sqlite, nss and nspr are
all folded together, because no shared library is actually linked in
db/sqlite3/src.