Change how C++ standard is handled for plugins

[sethhall]

Is this change valid for the cmake repo? The ZEEK_CXX_STD variable is properly propagated from zeek but I know these cmake scripts are used in other places too and I don't know if I'd break those if I tried to push for this change.

diff --git a/ZeekPluginCommon.cmake b/ZeekPluginCommon.cmake
index 6b12c75..900b258 100644
--- a/ZeekPluginCommon.cmake
+++ b/ZeekPluginCommon.cmake
@@ -24,7 +24,7 @@ macro (zeek_plugin_begin ns name)
     set(_plugin_bif_files "")
     set(_plugin_pac_args "")

-    target_compile_features(${_plugin_lib} PRIVATE cxx_std_17)
+    target_compile_features(${_plugin_lib} PRIVATE ${ZEEK_CXX_STD})
     set_target_properties(${_plugin_lib} PROPERTIES CXX_EXTENSIONS OFF)
 endmacro ()

[timwoj]

That should be ok. We try to enforce whatever standard Zeek is using across the board.