Enable building with JEMalloc

JEMalloc is an open source malloc library with good multi-threaded support.

Add --enable-jemalloc to configure.
Also allows --with-jemalloc-libraries to specify where the library is located.

If the library is not found, an error is reported.

Passes PR scripts:
https://buildbot.openinfosecfoundation.org/builders/ken-tilera-pcap
https://buildbot.openinfosecfoundation.org/builders/ken-tilera