Fixes crash with gtest when libc++ is enabled. Crash root cause
is missing "default" visibility for ThreadLocal class. ThreadLocal
class objects use typeinfo (typeid()) requiring "default" visibility.
Upstream gtest bug: https://github.com/google/googletest/issues/1207

diff -Nuar old/googletest/include/gtest/internal/gtest-port.h new/googletest/include/gtest/internal/gtest-port.h
--- old/googletest/include/gtest/internal/gtest-port.h
+++ new/googletest/include/gtest/internal/gtest-port.h
@@ -2027,7 +2027,7 @@
 
 // Implements thread-local storage on pthreads-based systems.
 template <typename T>
-class ThreadLocal {
+class GTEST_API_ ThreadLocal {
  public:
   ThreadLocal()
       : key_(CreateKey()), default_factory_(new DefaultValueHolderFactory()) {}