rpm: build optionally with jemalloc support

Add an option to build with jemalloc. jemalloc is available in Fedora
and EPEL. To build with jemalloc, do one of the following:

    $ cmake -DALLOCATOR=jemalloc $path_to_src
    $ make rpm

or

    $ mock --with=jemalloc rebuild $path_to_srpm

Signed-off-by: Niels de Vos <ndevos@redhat.com>