build: fix bsd build with gcc

BSD build with GCC was broken because it was checking for the
llvm_version variable on common.gypi, even though llvm wasn't
installed (or needed).

PR-URL: https://github.com/nodejs/node/pull/16737
Fixes: https://github.com/nodejs/node/issues/16257


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Refael Ackermann <refack@gmail.com>