Log Message:
Create a modifier PERL5_OPTIONS that specifies properties of the perl
installation needed when including perl5/buildlink3.mk.  The only
option currently supported is "threads", which implies that a perl
that supports threads is required.  The requirement is checked at
pre-install time using an INSTALL script template by both the package
build and the binary package.

Add PERL5_OPTIONS+=threads to both devel/p5-SDL and graphics/p5-GD
since those modules require a perl that supports threads.