Revert "[libcxx] Fix compile for BUILD_EXTERNAL_THREAD_LIBRARY"

This reverts commit 3b71f91558ff8b569199547efe800cb501c3cf94.

The commit is breaking some build bots.

GitOrigin-RevId: a19fd1aab519ccec18654f76a01b0345880c5200
diff --git a/include/__threading_support b/include/__threading_support
index f8add05..072c4c7 100644
--- a/include/__threading_support
+++ b/include/__threading_support
@@ -282,7 +282,7 @@
   bool operator()(chrono::nanoseconds __elapsed) const;
 };
 
-inline _LIBCPP_THREAD_ABI_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY
 bool __libcpp_timed_backoff_policy::operator()(chrono::nanoseconds __elapsed) const
 {
     if(__elapsed > chrono::milliseconds(128))
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index dcd53c8..2001c09 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -323,8 +323,7 @@
 
 
 if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
-  set(LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES
-      "${CMAKE_CURRENT_SOURCE_DIR}/../test/support/external_threads.cpp")
+  file(GLOB LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES ../test/support/external_threads.cpp)
 
   if (LIBCXX_ENABLE_SHARED)
     add_library(cxx_external_threads SHARED ${LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES})
@@ -338,8 +337,6 @@
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       OUTPUT_NAME   "c++external_threads"
   )
-
-  target_link_libraries(cxx_external_threads PRIVATE cxx-headers)
 endif()
 
 if (LIBCXX_INSTALL_LIBRARY)