Don't report missing PIE flags in buildd mode if GCC defaults to PIE

Don't report missing PIE flags in buildd mode if GCC defaults to PIE

Dpkg since 1.8.15 respects GCC's settings and doesn't inject PIE flags
if GCC already applies them internally. Therefore don't require PIE
flags if a recent enough dpkg version is used and GCC applies PIE on the
current architecture.