libcxx: Stop providing a definition of __GLIBC_PREREQ.

An application may determine whether the C standard library is glibc
by testing whether __GLIBC_PREREQ is defined. This breaks if libc++
provides its own definition. Instead, define our own macro in our
namespace with the desired semantics.

Differential Revision: https://reviews.llvm.org/D41892

llvm-svn: 322201