Unbreak build on FreeBSD 9.x amd64.

(Marked broken since r424405.)

Punch out the -pie used during link, which fixes linker errors.
(I'd tried setting -fPIE in CXXFLAGS before, which turned out not to
solve the problem - I won't dig deeper now less than eight weeks before
9.x goes unsupported.)

PR: 214331 (related)
Approved by: portmgr blanket (just fix it)