Fix build with systemtap/DTRACE providers on Fedora 21

The "dtrace" script version include in systemtap-sdt-devel-2.6-3
(part of Fedora 21) no longer ignores unknown command line
arguments, but will instead error out and refuse to run.
This patch adds a separate condition to node's gyp input so
that on Linux it will run dtrace without the -xnolibs
argument that trips it up on systemtap-std-devel-2.6-3.