Use system allocator when jemalloc is unavailable

Use system allocator when jemalloc is unavailable

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?