This commit adds support for systems where the jemalloc library is not
available as a package, here's the quick summary:
* On Linux - the jemalloc is usually available as a package, if
configured --without-jemalloc, the shim would be used around
malloc(), free(), realloc() and malloc_usable_size()
* On macOS - the jemalloc is available from homebrew or macports, if
configured --without-jemalloc, the shim would be used around
malloc(), free(), realloc() and malloc_size()
* On FreeBSD - the jemalloc is *the* system allocator, we just need
to check for <malloc_np.h> header to get access to non-standard API
* On NetBSD - the jemalloc is *the* system allocator, we just need to
check for <jemalloc/jemalloc.h> header to get access to non-standard
API
* On a system hostile to users and developers (read OpenBSD) - the
jemalloc API is emulated by using ((size_t *)ptr)[-1] field to hold
the size information. The OpenBSD developers care only for
themselves, so why should we care about speed on OpenBSD?
