Improve vpath support in plperl build

Improve vpath support in plperl build

Run xsubpp with the -output option instead of redirecting stdout.  That
ensures that the #line directives in the output file point to the right
place in a vpath build.  This in turn fixes an error in coverage builds
that it can't find the source files.

Refactor the makefile rules while we're here.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>