[libc++] Remove _LIBCPP_BUILDING_XXX macros, which are redundant since _LIBCPP_BUILDING_LIBRARY

Summary: As suggested by Marshall in https://reviews.llvm.org/D49914

Reviewers: mclow.lists, EricWF

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50008

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@338475 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/shared_mutex b/include/shared_mutex
index 9cb8152..a7735d6 100644
--- a/include/shared_mutex
+++ b/include/shared_mutex
@@ -129,7 +129,7 @@
 #include <__undef_macros>
 
 
-#if _LIBCPP_STD_VER > 11 || defined(_LIBCPP_BUILDING_SHARED_MUTEX)
+#if _LIBCPP_STD_VER > 11 || defined(_LIBCPP_BUILDING_LIBRARY)
 
 #include <__mutex_base>