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.

.gitignore

@@ -54,6 +54,13 @@ test/include/test/jemalloc_test_defs.h
 /test/integration/*.gcno
 /test/integration/*.out
 
+/test/integration/cpp/[A-Za-z]*
+!/test/integration/cpp/[A-Za-z]*.*
+/test/integration/cpp/*.[od]
+/test/integration/cpp/*.gcda
+/test/integration/cpp/*.gcno
+/test/integration/cpp/*.out
+
 /test/src/*.[od]
 /test/src/*.gcda
 /test/src/*.gcno