Merge branch 'develop':

Android + clang/libc++abi fixes.

include/boost/config/platform/linux.hpp

@@ -72,7 +72,7 @@
 // boilerplate code:
 #define BOOST_HAS_UNISTD_H
 #include <boost/config/posix_features.hpp>
-#ifdef __USE_GNU
+#if defined(__USE_GNU) && !defined(__ANDROID__) && !defined(ANDROID)
 #define BOOST_HAS_PTHREAD_YIELD
 #endif
 

include/boost/config/stdlib/libcpp.hpp

@@ -75,6 +75,14 @@
 #define BOOST_NO_STD_MESSAGES
 #endif
 
+#if (_LIBCPP_VERSION <= 1101) && !defined(BOOST_NO_CXX11_THREAD_LOCAL)
+// This is a bit of a sledgehammer, because really it's just libc++abi that has no
+// support for thread_local, leading to linker errors such as
+// "undefined reference to `__cxa_thread_atexit'".  It is fixed in the
+// most recent releases of libc++abi though...
+#  define BOOST_NO_CXX11_THREAD_LOCAL
+#endif
+
 #if defined(__has_include)
 #if !__has_include(<shared_mutex>)
 #  define BOOST_NO_CXX14_HDR_SHARED_MUTEX

test/boost_no_constexpr.ipp

@@ -32,7 +32,13 @@ private:
 
 template<int> struct X { };
 
+
+#if __cplusplus < 201400L
+// Some C++14 compilers reject this (clang), some C++11 compilers reject "constexpr const" (GCC-4.6)
+constexpr A a = 42;
+#else
 constexpr const A a = 42;
+#endif
 
 X<a> xx; // OK: unique conversion to int