perl plugin: Fixed the "EnableDebugger after LoadPlugin" warning.

perl plugin: Fixed the "EnableDebugger after LoadPlugin" warning.

For some strange reason my original patch (perl plugin: Warn if
"EnableDebugger" has been used after "LoadPlugin".) has been applied at the
wrong position (it has been applied to perl_config_includedir() instead of
perl_config_enabledebugger() - I have absolutely no clue why though).

Also, the check for the precondition of this warning has been fixed. In some
cases aTHX does not seem to be set, even though the Perl interpreter has
already been initialized. Now, perl_threads is used to check for that
condition.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>