jemalloc cpp new/delete bindings

Adds cpp bindings for jemalloc, along with necessary autoconf settings.
This is mostly to add sized deallocation support, which can't be added
from C directly.  Sized deallocation is ~10% microbench improvement.

* Import ax_cxx_compile_stdcxx.m4 from the autoconf repo, seems like the
  easiest way to get c++14 detection.
* Adds various other changes, like CXXFLAGS, to configure.ac.
* Adds new rules to Makefile.in for src/jemalloc-cpp.cpp, and a basic
  unittest.
* Both new and delete are overridden, to ensure jemalloc is used for
  both.
* TODO future enhancement of avoiding extra PLT thunks for new and
  delete - sdallocx and malloc are publicly exported jemalloc symbols,
  using an alias would link them directly.  Unfortunately, was having
  trouble getting it to play nice with jemalloc's namespace support.

Testing:
Tested gcc 4.8, gcc 5, gcc 5.2, clang 4.0.  Only gcc >= 5 has sized
deallocation support, verified that the rest build correctly.

Tested mac osx and Centos.

Tested --with-jemalloc-prefix and --without-export.

This resolves #202.

test/include/test/jemalloc_test.h.in

@@ -1,3 +1,7 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <limits.h>
 #ifndef SIZE_T_MAX
 #  define SIZE_T_MAX	SIZE_MAX
@@ -47,7 +51,8 @@
  * expose the minimum necessary internal utility code (to avoid re-implementing
  * essentially identical code within the test infrastructure).
  */
-#elif defined(JEMALLOC_INTEGRATION_TEST)
+#elif defined(JEMALLOC_INTEGRATION_TEST) || \
+    defined(JEMALLOC_INTEGRATION_CPP_TEST)
 #  define JEMALLOC_MANGLE
 #  include "jemalloc/jemalloc@install_suffix@.h"
 #  include "jemalloc/internal/jemalloc_internal_defs.h"
@@ -161,3 +166,7 @@ static const bool config_debug =
 	if (!(e))							\
 		not_implemented();					\
 } while (0)
+
+#ifdef __cplusplus
+}
+#endif

test/include/test/test.h

@@ -8,8 +8,8 @@
 		char message[ASSERT_BUFSIZE];				\
 		malloc_snprintf(prefix, sizeof(prefix),			\
 		    "%s:%s:%d: Failed assertion: "			\
-		    "(%s) "#cmp" (%s) --> "				\
-		    "%"pri" "#neg_cmp" %"pri": ",			\
+		    "(%s) " #cmp " (%s) --> "				\
+		    "%" pri " " #neg_cmp " %" pri ": ",			\
 		    __func__, __FILE__, __LINE__,			\
 		    #a, #b, a_, b_);					\
 		malloc_snprintf(message, sizeof(message), __VA_ARGS__);	\