Squashed the commits of the topic/robin/dynamic-plugins branch.
Updating the code for generating dynamic plugins.

Includes various cleanup and layout changes, support for "make
install", and support for Darwin.
Adding Bro's {C,CXX}_FLAGS when building dynamic plugins out of tree.
Support for building in-tree plugins either statically or dynamically.

Default remains static, one can switch a plugin to dynamic by adding
SET(BRO_PLUGIN_BUILD_DYNAMIC TRUE) to the beginning of its CMakeIndex.txt
Fixing link order.
Merge remote-tracking branch 'origin/master' into topic/robin/dynamic-plugins-2.3

Conflicts:
	BifCl.cmake
Fxing merge relict.
Small tweaks to plugin support.
Linker fix for Darwin,

Not sure when/why this changed ...
Fix for building plugins in debug mode.
Merge branch 'topic/robin/dynamic-plugins-2.3' of ssh://git.bro.org/cmake into topic/robin/dynamic-plugins-2.3
Fix for plugin support on FreeBSD.
Show hopefully fix linking on newer FreeBSDs.
Adding macro to link external library to plugin.
Fix missing target in dynamic plugin mode.
Adding an option to change the installation directory for a dynamic
plugin.

This is primarily for testing purposes.
Merge remote-tracking branch 'origin/topic/robin/dynamic-plugins-2.3'

* origin/topic/robin/dynamic-plugins-2.3:
  Adding an option to change the installation directory for a dynamic plugin.
  Fix missing target in dynamic plugin mode.
  Adding macro to link external library to plugin.
  Show hopefully fix linking on newer FreeBSDs.
  Fix for plugin support on FreeBSD.
  Fix for building plugins in debug mode.
  Linker fix for Darwin,
  Small tweaks to plugin support.
  Fxing merge relict.
  Fixing link order.
  Support for building in-tree plugins either statically or dynamically.
  Adding Bro's {C,CXX}_FLAGS when building dynamic plugins out of tree.
  Updating the code for generating dynamic plugins.
  Squashed the commits of the topic/robin/dynamic-plugins branch.
Fix Bro install prefix in CMake summary output for dynamic plugins.