[llvm-dev] Bootstrapping with in-tree libcxx

[Carsten Mattner via llvm-dev]

When I tried to build with -DLLVM_ENABLE_LIBCXX, I found out that
it doesn't try to use the in-tree libcxx but looks for one built and installed
in the system.

Since building all of llvm required CXX and using libcxx with the same
version as the rest of llvm makes sense, I would certainly appreciate
an option to build the in-tree libcxx as a first step and use it in when
building llvm, lld, lldb, clang, ...

Since llvm already builds many other DSOs it needs at runtime, adding
libcxx is reasonable. More so if we consider that not all systems package
libcxx at the same version as clang and llvm. For instance, Arch Linux
has libcxx 3.8.0 but llvm and clang 3.9.0.