Fix boost build with unified headers.

boost (1.64.0 at least) assumes that if you have POSIX_MADV_NORMAL, you have
posix_madvise. With unified headers, this isn't true. Rather than make life
harder for projects that don't use configure, just make it so. We already
applied similar workarounds for epoll_create1 and inotify_init1.

Bug: https://github.com/android-ndk/ndk/issues/395
Test: built boost (long story!)
Change-Id: I5d2d8de7b30921dde913251d35dcd249a2876f94