Adding new modules for supporting plugins.

    - BroPlugin: CMake macros to define plugins.
    - BifCl:     Factoring bifcl functions out of main CMakeIndex.txt
    - BinPAC:    Factoring binpac functions out of main CMakeIndex.txt
A further variant of the bifcl magic to build/include *.bif at other
locations that the root directory.

This still needs a bit of cleanup, we can probably factor some of this
out.
Macro to define plugin dependencies.
Updates to BifCL/BinPAC/BroPlugin modules for fixing dependencies.
Merge remote-tracking branch 'origin/master' into topic/robin/plugin
Cleanup and introducing bro_HAVE_OBJECT_LIBRARIES.

The variable switches between object and static libraries for plugins
and subdirs.