Better <bits/c++config.h> detection

Summary:
When worrying about only 2 cpp libraries, detection of "is this libstdc++ or libc++" works, but when throwing in a third option (msvc) this becomes a very brittle way of detecting features.  Also depending on how clang is used it may or may not have this header about

Changed to instead use configuration value for "do we have this header file" for including bits/c++config.h and changed the logic to be identical in all three use cases

Test Plan: fbmake runtests

Reviewed By: njormrod@fb.com

FB internal diff: D1291547

folly/FBString.h

@@ -63,8 +63,10 @@
 #include <type_traits>
 #include <algorithm>
 
-// libc++ doesn't provide this header
-#ifndef _LIBCPP_VERSION
+#include "folly/Portability.h"
+
+// libc++ doesn't provide this header, nor does msvc
+#ifdef FOLLY_HAVE_BITS_CXXCONFIG_H
 // This file appears in two locations: inside fbcode and in the
 // libstdc++ source code (when embedding fbstring as std::string).
 // To aid in this schizophrenic use, two macros are defined in

folly/Range.h

@@ -31,8 +31,8 @@
 #include <string>
 #include <type_traits>
 
-// libc++ doesn't provide this header
-#if !FOLLY_USE_LIBCPP
+// libc++ doesn't provide this header, nor does msvc
+#ifdef FOLLY_HAVE_BITS_CXXCONFIG_H
 // This file appears in two locations: inside fbcode and in the
 // libstdc++ source code (when embedding fbstring as std::string).
 // To aid in this schizophrenic use, two macros are defined in

folly/Traits.h

@@ -26,8 +26,8 @@
 
 #include "folly/Portability.h"
 
-// libc++ doesn't provide this header
-#if !FOLLY_USE_LIBCPP
+// libc++ doesn't provide this header, nor does msvc
+#ifdef FOLLY_HAVE_BITS_CXXCONFIG_H
 // This file appears in two locations: inside fbcode and in the
 // libstdc++ source code (when embedding fbstring as std::string).
 // To aid in this schizophrenic use, two macros are defined in

folly/configure.ac

@@ -52,7 +52,7 @@ AX_BOOST_SYSTEM
 
 # Checks for header files.
 AC_HEADER_STDC
-AC_CHECK_HEADERS([fcntl.h features.h inttypes.h limits.h stdint.h stdlib.h string.h sys/time.h unistd.h mutex.h malloc.h emmintrin.h byteswap.h bits/functexcept.h])
+AC_CHECK_HEADERS([fcntl.h features.h inttypes.h limits.h stdint.h stdlib.h string.h sys/time.h unistd.h mutex.h malloc.h emmintrin.h byteswap.h bits/functexcept.h bits/c++config.h])
 
 AC_CHECK_HEADER(double-conversion/double-conversion.h, [], [AC_MSG_ERROR(
                 [Couldn't find double-conversion.h, please download from \