latest VM images fail to compile for ARM

[Ed Maste]

>> I suspect the culprit is:
>>
>>    r272849 emaste: Build gperf only if we’re using g++ (not clang++)

Sorry I didn't notice this thread when this problem was first identified.

> Tim, should I open a bug report for this or did you do it already?

I believe this should be fixed as of r275602 [1]

As reported before be sure to clear out the obj directory before
trying to build with the fix (or when using the workaround of manually
building and installing gperf first).

[1] https://svnweb.freebsd.org/changeset/base/275602