diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 865da86d7cac065fefa430f08bd352ed4c66fa15..40351ddcfaa14e0ed253031100a04f9d3323df61 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -48,7 +48,7 @@ will be expected to conform to the style outlined
 
 *   Don't fix code style and formatting unless you are already changing that
     line to address an issue. Formatting of modified lines may be done using
-    `git clang-format`. PRs with irrelevant changes won't be merged. If
+   `git clang-format`. PRs with irrelevant changes won't be merged. If
     you do want to fix formatting or style, do that in a separate PR.
 
 *   Unless your PR is trivial, you should expect there will be reviewer comments
diff --git a/absl/base/internal/low_level_alloc.cc b/absl/base/internal/low_level_alloc.cc
index 08f89ea9ee5437aceb13cabd1a5a72b98ac550ba..8e2f9c98798b4f647a471001811b58ff92070021 100644
--- a/absl/base/internal/low_level_alloc.cc
+++ b/absl/base/internal/low_level_alloc.cc
@@ -30,6 +30,7 @@
 #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
 
 #ifndef _WIN32
+#include <pthread.h>
 #include <signal.h>
 #include <sys/mman.h>
 #include <unistd.h>
diff --git a/absl/base/internal/tsan_mutex_interface.h b/absl/base/internal/tsan_mutex_interface.h
index a1303e67c8f513cf6f7f3290d091aa3f3e1aa994..6bb4faedb879bb84bbab8b36c67bf5453ac8bfac 100644
--- a/absl/base/internal/tsan_mutex_interface.h
+++ b/absl/base/internal/tsan_mutex_interface.h
@@ -19,7 +19,22 @@
 #ifndef ABSL_BASE_INTERNAL_TSAN_MUTEX_INTERFACE_H_
 #define ABSL_BASE_INTERNAL_TSAN_MUTEX_INTERFACE_H_
 
-#ifdef THREAD_SANITIZER
+// ABSL_INTERNAL_HAVE_TSAN_INTERFACE
+// Macro intended only for internal use.
+//
+// Checks whether LLVM Thread Sanitizer interfaces are available.
+// First made available in LLVM 5.0 (Sep 2017).
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
+#error "ABSL_INTERNAL_HAVE_TSAN_INTERFACE cannot be directly set."
+#endif
+
+#if defined(THREAD_SANITIZER) && defined(__has_include)
+#if __has_include(<sanitizer/tsan_interface.h>)
+#define ABSL_INTERNAL_HAVE_TSAN_INTERFACE 1
+#endif
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
 #include <sanitizer/tsan_interface.h>
 
 #define ABSL_TSAN_MUTEX_CREATE __tsan_mutex_create