Summary:
This patch contains the following changes:
1. Require that libc++ can find a LLVM source directory. This is done the same way as `libc++abi` currently does.
2. Cleanup ugly configuration code in CMakeLists.txt by using `add_flags`, `add_flags_if`, and `add_flags_if_supported` macros.
The goals for this patch are:
1. Help libc++ be more consistent with how LLVM handles CMake options (see PR23670 PR23671).
2. Make it easier to use sanitizers using the `LLVM_USE_SANITIZER` option.
3. Make libc++'s CMakeLists.txt file easier to understand and change.
4. Move towards allowing libc++ to create Sphinx documentation (see http://efcs.ca/libcxx-docs).
5. Move towards allowing libc++ to use other LLVM utilities such as `not` and `FileCheck`.
Reviewers: mclow.lists, jroelofs, danalbert
Subscribers: cfe-commits
Differential Revision: http://reviews.llvm.org/D11308
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@243503 91177308-0d34-0410-b5e6-96231b3b80d8
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