This adds "--enable-tcmalloc" and "--disable-tcmalloc" to allow linking
to libtcmalloc from gperftools.

tcmalloc is a malloc implementation that works well with threads and is
fast, so it is good for performance.

It is disabled by default, because the MALLOC_PERTURB_ flag we use in
tests doesn't work with tcmalloc. However we can enable tcmalloc
specific heap checker and profilers later.

An IOPS gain can be observed with virtio-blk-dataplane, other parts of
QEMU will directly benefit from it as well:

==========================================================
                       glibc malloc
----------------------------------------------------------
rw         bs         iodepth    bw     iops       latency
read       4k         1          150    38511      24
----------------------------------------------------------

==========================================================
                         tcmalloc
----------------------------------------------------------
rw         bs         iodepth    bw     iops       latency
read       4k         1          156    39969      23
----------------------------------------------------------
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <1427338992-27057-1-git-send-email-famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>