cirrus: Add smoke testing for builtin plugins

This adds two example plugins within testing/builtin-plugins/Files:

* protocol-plugin copied over from testing/btest/plugins/protocol-plugin

* py-lib-plugin that embeds Python to have a dependency on an external
  shared library which was already available in CI and fun to use, too.

Closes #2837

testing/builtin-plugins/Files/py-lib-plugin/CMakeLists.txt

@@ -0,0 +1,34 @@
+cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
+
+find_package(PythonLibs REQUIRED)
+
+file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/VERSION" VERSION LIMIT_COUNT 1)
+
+string(REGEX REPLACE "[.-]" " " version_numbers ${VERSION})
+separate_arguments(version_numbers)
+list(GET version_numbers 0 VERSION_MAJOR)
+list(GET version_numbers 1 VERSION_MINOR)
+list(GET version_numbers 2 VERSION_PATCH)
+
+# Our plugin source and scripts are in a subdirectory
+set(BRO_PLUGIN_BASE "${CMAKE_CURRENT_SOURCE_DIR}/plugin")
+
+include(ZeekPlugin)
+
+include_directories(BEFORE ${PYTHON_INCLUDE_DIR})
+
+zeek_plugin_begin(Zeek PyLib)
+
+file(GLOB cc_files RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}" "plugin/src/*.cc")
+foreach(file ${cc_files})
+    zeek_plugin_cc(${file})
+endforeach ()
+
+file(GLOB bif_files RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}" "plugin/src/*.bif")
+foreach(file ${bif_files})
+    zeek_plugin_bif(${file})
+endforeach ()
+
+zeek_plugin_dist_files(README VERSION)
+zeek_plugin_link_library(${PYTHON_LIBRARY})
+zeek_plugin_end()