Prior to this patch, we would generate a fancy <config> header by
concatenating <config_site> and <__config>. This complexifies the
build system and also increases the difference between what's tested
and what's actually installed.
This patch removes that complexity and instead simply installs <config_site>
alongside the libc++ headers. <config_site> is then included by <__config>,
which is much simpler.
Doing this also opens the door to having different <__config_site> headers
depending on the target, which was impossible before.