FreeBSD: disable buggy -fstack-protector-all

FreeBSD ships an old gcc 4.2.1 which generates
bogus code, e.g. getsockopt() call returns
struct xucred with bogus values, which doesn't even
allow to connect to libvirtd:

error: Failed to find group record for gid '1284660778': No error: 0

So roll back to just -fstack-protector on FreeBSD.

(cherry picked from commit cc7cd623)

m4/virt-compile-warnings.m4

@@ -183,6 +183,13 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
        gl_WARN_ADD([-fstack-protector-all])
        gl_WARN_ADD([--param=ssp-buffer-size=4])
        ;;
+       *-*-freebsd*)
+       dnl FreeBSD ships old gcc 4.2.1 which doesn't handle
+       dnl -fstack-protector-all well
+       gl_WARN_ADD([-fstack-protector])
+
+       gl_WARN_ADD([-Wno-unused-command-line-argument])
+       ;;
     esac
     gl_WARN_ADD([-fexceptions])
     gl_WARN_ADD([-fasynchronous-unwind-tables])