build: disable dtrace on non-Linux builds

build: disable dtrace on non-Linux builds

Using dtrace (and systemtap in general) is Linux-specific.
Running ./autobuild.sh shows that attempting a cross-build to
target mingw was mistakenly trying to build dtrace code, merely
because it was present on the compilation host.

* configure.ac (with_dtrace): Don't attempt to use dtrace when
doing a cross-build hosted on Linux but targetting elsewhere.
Reported by Daniel P. Berrange.