[libc++] Detect C11 features on non-Clang compilers

Summary:
The macros were inside `#if defined(_LIBCPP_COMPILER_CLANG)`, which means
we would never detect C11 features on non-Clang compilers. According to
Marshall Clow, this is not the intended behavior.

Reviewers: mclow.lists, EricWF

Subscribers: krytarowski, christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50748

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@339741 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index 4c3b25b..cd2a135 100644
--- a/include/__config
+++ b/include/__config
@@ -328,6 +328,28 @@
 #  define _LIBCPP_NO_CFI
 #endif
 
+#if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
+#  if defined(__FreeBSD__)
+#    define _LIBCPP_HAS_QUICK_EXIT
+#    define _LIBCPP_HAS_C11_FEATURES
+#  elif defined(__Fuchsia__)
+#    define _LIBCPP_HAS_QUICK_EXIT
+#    define _LIBCPP_HAS_C11_FEATURES
+#  elif defined(__linux__)
+#    if !defined(_LIBCPP_HAS_MUSL_LIBC)
+#      if _LIBCPP_GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
+#        define _LIBCPP_HAS_QUICK_EXIT
+#      endif
+#      if _LIBCPP_GLIBC_PREREQ(2, 17)
+#        define _LIBCPP_HAS_C11_FEATURES
+#      endif
+#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
+#      define _LIBCPP_HAS_QUICK_EXIT
+#      define _LIBCPP_HAS_C11_FEATURES
+#    endif
+#  endif // __linux__
+#endif
+
 #if defined(_LIBCPP_COMPILER_CLANG)
 
 // _LIBCPP_ALTERNATE_STRING_LAYOUT is an old name for
@@ -430,28 +452,6 @@
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #endif
 
-#if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
-#  if defined(__FreeBSD__)
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    define _LIBCPP_HAS_C11_FEATURES
-#  elif defined(__Fuchsia__)
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    define _LIBCPP_HAS_C11_FEATURES
-#  elif defined(__linux__)
-#    if !defined(_LIBCPP_HAS_MUSL_LIBC)
-#      if _LIBCPP_GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
-#        define _LIBCPP_HAS_QUICK_EXIT
-#      endif
-#      if _LIBCPP_GLIBC_PREREQ(2, 17)
-#        define _LIBCPP_HAS_C11_FEATURES
-#      endif
-#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
-#      define _LIBCPP_HAS_QUICK_EXIT
-#      define _LIBCPP_HAS_C11_FEATURES
-#    endif
-#  endif // __linux__
-#endif
-
 #if !(__has_feature(cxx_noexcept))
 #define _LIBCPP_HAS_NO_NOEXCEPT
 #endif