Updated specfile and configure script for libdir

https://github.com/bro/broccoli/pull/1

Package maintainers and those that would otherwise compile from source
were unable to specify the installation directory of architecture
dependent libraries. Namely, many distributions use lib64/ versus lib/
for the installation of architecture dependent library archives.

* Add new 'configure' option, --libdir
* Defaults to old behavior of "$prefix/lib"
* Follows Kitware example for ProjectConfig.cmake on wiki