clang++-libc++ with clang6 and libcxxabi does support thread_local about config

[PlacidBox]

Did some digging and that commit to libcxxabi looks like it was included in clang4. Here's the patch I'm currently using. It might break some users that aren't using libcxxabi though.. not really too sure on the best way to handle this.

--- boost/config/stdlib/libcpp.hpp	2018-07-26 10:09:30.091065048 +1000
+++ boost/config/stdlib/libcpp.hpp	2018-07-26 10:14:24.641990321 +1000
@@ -113,10 +113,16 @@
 #  define BOOST_NO_CXX11_THREAD_LOCAL
 #endif
 
-#if defined(__linux__) && !defined(BOOST_NO_CXX11_THREAD_LOCAL)
+#if defined(__linux__) && _LIBCPP_VERSION < 4000 && !defined(BOOST_NO_CXX11_THREAD_LOCAL)
 // After libc++-dev is installed on Trusty, clang++-libc++ almost works,
 // except uses of `thread_local` fail with undefined reference to
 // `__cxa_thread_atexit`.
+//
+// clang's libc++abi provides an implementation by deferring to the glibc
+// implementation, which may or may not be available (it is not on Trusty).
+// clang 4's libc++abi will provide an implementation if one is not in glibc
+// though, so thread local support should work with clang 4 and above as long
+// as libc++abi is linked in.
 #  define BOOST_NO_CXX11_THREAD_LOCAL
 #endif
 

[jzmaddock]

Confirmed as working with Ubuntu clang-6 packages at least.

[jzmaddock]

Spoke too soon.... failed CI testing with clang-5. Updating the CI tests and waiting for them to cycle...

[jzmaddock]

CI is passing now, not sure if we have all the wrinkles sorted, but it's better than it was.