libscript-perl: support building without Dusemultiplicity, but in this case fobid multiple states #3

Typical builds of Perl are built without the -Dusemultiplicity flag, which is necessary for loading multiple VMs in parallel in a single process. Since requiring users to rebuild Perl complicates the use of the library a lot, it would be nice to, instead of failing, detecting if -Dusemultiplicity is available and just disable the feature of multilple VMs.

To do this, one would have to:

• tweak configure.ac to make it set an AC_DEFINE instead of failing with AC_MSG_ERROR
• using the define (reachable via config.h) to detect if usemultiplicity is available in the code of the libscript-perl plugin
• when usemultiplicity is not available, return failure if one tries to init the plugin more than once.