• Fix build on 10.1: disable -fno-defer-pop flag which is not known by clang

Makefile says this flag is crucial for gcc to disable certain
optimization, but I assume clang doesn't have this optimization as
it doesn't support the flag. clang >= 3.6 handles unknown flags
with a mere warnings, so the modification is only needed for clang < 3.6

Submitted by: pkg-fallout
Approved by: portmgr blanket