[FreeBSD] Add -fno-omit-frame-pointer to CFLAGS to fix segfault in backtrace().

Looks like at certain optimization levels, gcc omits frame pointer which leads
to segfault in backtrace(). Fix is to add explicit -fno-omit-frame-pointer to
always get frame pointers.

Signed-off-by: Sachin Bhamare <sbhamare@panasas.com>