Pick up GTEST_API_ definition in gtest/internal/custom/gtest-port.h.

This makes it possible for a port to define the specifier used for
exported symbols without having to change Google Test.

googletest/include/gtest/internal/gtest-port.h

@@ -930,6 +930,11 @@ using ::std::tuple_size;
 
 #endif  // GTEST_HAS_SEH
 
+// GTEST_API_ qualifies all symbols that must be exported. The definitions below
+// are guarded by #ifndef to give embedders a chance to define GTEST_API_ in
+// gtest/internal/custom/gtest-port.h
+#ifndef GTEST_API_
+
 #ifdef _MSC_VER
 # if GTEST_LINKED_AS_SHARED_LIBRARY
 #  define GTEST_API_ __declspec(dllimport)
@@ -940,9 +945,11 @@ using ::std::tuple_size;
 # define GTEST_API_ __attribute__((visibility ("default")))
 #endif // _MSC_VER
 
+#endif // GTEST_API_
+
 #ifndef GTEST_API_
 # define GTEST_API_
-#endif
+#endif // GTEST_API_
 
 #ifdef __GNUC__
 // Ask the compiler to never inline a given function.