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.

configure.ac

@@ -269,9 +269,10 @@ if test "$prefix" = "/usr" && test "$sysconfdir" = '${prefix}/etc' ; then
 fi
 
 dnl Make some notes about which OS we're compiling for, as the lxc and qemu
-dnl drivers require linux headers, while storage_mpath and nwfilter are also
-dnl linux specific.  The "network" and storage_fs drivers are known to not
-dnl work on MacOS X presently, so we also make a note if compiling for that
+dnl drivers require linux headers, and storage_mpath, dtrace, and nwfilter
+dnl are also linux specific.  The "network" and storage_fs drivers are known
+dnl to not work on MacOS X presently, so we also make a note if compiling
+dnl for that
 
 with_linux=no with_osx=no
 case $host in
@@ -288,6 +289,7 @@ if test $with_linux = no; then
     then
         with_qemu=no
     fi
+    with_dtrace=no
 fi
 
 AM_CONDITIONAL([WITH_LINUX], [test "$with_linux" = "yes"])