Tk::MMutil: use $^X instead of $self->{PERL}

Since ExtUtils::MakeMaker 6.99_10, the PERL object member is quoted.
This caused the subsequent extraction commands to be run with an
invalid command (the perl binary *with* quotes). Unfortunately
this system() call failed silently, so the real problem showed
up in a strange dependency problem, see
https://rt.cpan.org/Ticket/Display.html?id=100044

Instead of $self->{PERL}, now simply $^X is used. Also, the
extraction commands do not fail silently anymore.

Reference to the EUMM ticket:
https://rt.cpan.org/Ticket/Display.html?id=100159