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.

bin/jemalloc-config.in

@@ -18,6 +18,7 @@ Options:
   --cc         : Print compiler used to build jemalloc.
   --cflags     : Print compiler flags used to build jemalloc.
   --cppflags   : Print preprocessor flags used to build jemalloc.
+  --cxxflags   : Print C++ compiler flags used to build jemalloc.
   --ldflags    : Print library flags used to build jemalloc.
   --libs       : Print libraries jemalloc was linked against.
 EOF
@@ -67,6 +68,9 @@ case "$1" in
 --cppflags)
 	echo "@CPPFLAGS@"
 	;;
+--cxxflags)
+	echo "@CXXFLAGS@"
+	;;
 --ldflags)
 	echo "@LDFLAGS@ @EXTRA_LDFLAGS@"
 	;;