haproxy: tweak thread activation

haproxy: tweak thread activation

If threads are enabled, as described in include/common/hathreads.h,
haproxy uses __sync_*_4 intrisics if gcc < 4.7. Otherwise, haproxy
uses __atomic_ intrinsics.

As a result, instead of adding a dependency on BR2_TOOLCHAIN_HAS_SYNC_4
and BR2_TOOLCHAIN_HAS_ATOMIC in Config.in, enable threads only if the
dependencies are fulfilled in haproxy.mk

Fixes:
 - http://autobuild.buildroot.org/results/7f24873ecdd9246c95c03bb8d2fcd4c16c488c6c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>