configure: respect the --dest-os flag consistently

Consider a user on his Mac, who wants to cross-compile for his Linux ARM device:

    ./configure --dest-cpu=arm --dest-os=linux

Before this patch, for example, DTrace probes would incorrectly attempt to be
enabled because the configure script is running on a MacOS machine, even though
we're trying to compile a binary for `linux`.

With this patch, the `--dest-os` flag is respected throughout the configure
script, thus leaving DTrace probes disabled in this cross-compiling scenario.

P.S. this PR is targeted against `v0.10` and it should be fine there IMO, but if someone thinks it should go into master instead then speak up.