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/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__);	\