[libc++] Cleanup and document <__threading_support>
Summary:
This patch attempts to clean up the macro configuration mess in `<__threading_support>`, specifically the mess involving external threading variants. Additionally this patch adds design documentation for `<__threading_support>` and the configuration macros it uses.
The primary change in this patch is separating the idea of an "external API" provided by `<__external_threading>` and the idea of having an external threading library. Now `_LIBCPP_HAS_THREAD_API_EXTERNAL` means that libc++ should use `<__external_threading>` and that the header is expected to exist. Additionally the new macro `_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL` is now used to configure for using an "external library" with the default threading API.
Reviewers: compnerd, rmaprath
Subscribers: smeenai, cfe-commits, mgorny
Differential Revision: https://reviews.llvm.org/D28316
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291275 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config_site.in b/include/__config_site.in
index f2a1986..667b4e9 100644
--- a/include/__config_site.in
+++ b/include/__config_site.in
@@ -21,6 +21,7 @@
#cmakedefine _LIBCPP_HAS_MUSL_LIBC
#cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
#cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
+#cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
#cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
#endif // _LIBCPP_CONFIG_SITE