libtraceevent: Allow custom libdir path

libtraceevent: Allow custom libdir path

When I use prefix=/usr and try to install libtraceevent in my laptop it
tries to install in /usr/lib64. I am not having any folder as /usr/lib64
and also the debian policy doesnot allow installing in /usr/lib64. It
should be in /usr/lib/x86_64-linux-gnu/.

Quote: No package for a 64 bit architecture may install files in
	/usr/lib64/ or in a subdirectory of it.

ref: https://www.debian.org/doc/debian-policy/ch-opersys.html

Make it more flexible by allowing to mention libdir_relative while
installing so that distros can mention the path according to their
policy or use the default one.

Signed-off-by: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20191207111440.6574-1-sudipm.mukherjee@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>