Commit a workaround for archs with no clang in base. This avoids
the warning '"clang --version" returned non-zero status' when doing
operations such as 'make describe'.

I have been running with this patch on sparc64 for quite some time.
It has also been tested on powerpc64, and for harmlessness on amd64.

Arguably, this is a something missing in the infrastructure.

Approved by: portmgr (tier-2 blanket)