jemalloc (5.2.1 at least) cannot compile in C++ on macOS SDK, due
to conflicts on exception specification.

I found a way to suppress particular kind of warnings via pragmas.

* Otherwise it might segfault, since C has no idea of the type of varargs,
  and the C code must assume all varargs are VALUE.

We would like to skip this extension library when libstdc++ is missing.
To avoid such situation let's use try_link instead of try_compile.

The `cxx.try_compile` command in this file kicks `cxx.have_devel?`
internally, which recursively calls `cxx.try_link` with a different
source code.  We don't want that happen (the source code compiled in
this file must be the first one).  We need to fake the system.

Was my mistake to put return here.

C++ keyword `nullptr` represents a null pointer (note also that NULL is
an integer in C++ due to its design flaw).  Its type is `std::nullptr_t`,
defined in <cstddef> standard header.  Why not support it when the
backend implementation can take a null pointer as an argument.