Detect libc++ in configure.ac.

Summary:
- Explicitly detect use of libc++ in configure.ac and reflect this in
folly-config.h. Do this rather than looking for to pop up organically
_LIBCPP_VERSION since this only exists if we've included some libc++
header files prior to refernce. This isn't always the case, e.g. in
Portability.h.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: simpkins@fb.com

FB internal diff: D1002958

folly/Portability.h

@@ -121,7 +121,7 @@ struct MaxAlign { char c; } __attribute__((aligned));
 // It turns out that GNU libstdc++ and LLVM libc++ differ on how they implement
 // the 'std' namespace; the latter uses inline namepsaces. Wrap this decision
 // up in a macro to make forward-declarations easier.
-#ifdef _LIBCPP_VERSION
+#if FOLLY_USE_LIBCPP
 #define FOLLY_NAMESPACE_STD_BEGIN     _LIBCPP_BEGIN_NAMESPACE_STD
 #define FOLLY_NAMESPACE_STD_END       _LIBCPP_END_NAMESPACE_STD
 #else

folly/configure.ac

@@ -78,6 +78,15 @@ AC_COMPILE_IFELSE(
    AC_DEFINE([OVERRIDE], [],
              [Define to "override" if the compiler supports C++11 "override"])]
 )
+AC_COMPILE_IFELSE(
+  [AC_LANG_SOURCE[
+    #include <type_traits>
+    #if !_LIBCPP_VERSION
+    #error No libc++
+    #endif
+    void func() {}]
+  ],
+  [AC_DEFINE([USE_LIBCPP], [1], [Define to 1 if we're using libc++.])])
 
 # Check for clock_gettime(2). This is not in an AC_CHECK_FUNCS() because we
 # want to link with librt if necessary.

folly/test/ScopeGuardTest.cpp

@@ -15,6 +15,7 @@
  */
 
 #include "folly/ScopeGuard.h"
+#include "folly/Portability.h"
 
 #include <gflags/gflags.h>
 #include <gtest/gtest.h>
@@ -126,7 +127,7 @@ TEST(ScopeGuard, GuardException) {
       throw std::runtime_error("destructors should never throw!");
     });
   },
-#ifdef _LIBCPP_VERSION
+#if FOLLY_USE_LIBCPP
   "terminate called throwing an exception"
 #else
   "destructors should never throw"