Rationalize visibility.

If it's C or POSIX, it's in. If it's BSD or GNU, it's guarded by __USE_BSD
or __USE_GNU.

Bug: https://code.google.com/p/android/issues/detail?id=194631
Change-Id: Ife51a21c2b37b060db56780d29c929805b199cb6
diff --git a/tests/string_posix_strerror_r_test.cpp b/tests/string_posix_strerror_r_test.cpp
index ae3b41a..596684b 100644
--- a/tests/string_posix_strerror_r_test.cpp
+++ b/tests/string_posix_strerror_r_test.cpp
@@ -20,7 +20,7 @@
 #if defined(__GLIBC__)
 
 // At the time of writing, libcxx -- which is dragged in by gtest -- assumes
-// declarations from glibc of things that aren't available without __USE_GNU.
+// declarations from glibc of things that aren't available without _GNU_SOURCE.
 // This means we can't even build this test (which is a problem because that
 // means it doesn't get included in CTS).
 // For glibc 2.15, the symbols in question are: