t/xs.t to skip some if futile.
t/xs.t to test PERL="perl -w" exercising space-quoting.
Enable PERL=>"$^X -Iinc" to work again.
Merge pull request #150 from mohawk2/perlmacrospace

Enable PERL=>"$^X -Iinc" to work again.
Simulate Module::Install $(PERL) changing.
Invert logic to avoid "unless/else".
Handle Module::Install $(PERL) Makefile editing.
Merge pull request #151 from mohawk2/perlmacro-modinstall

Handle Module::Install $(PERL) Makefile editing.