Update ax_boost_chrono.m4

Hi, i known building system works well now, but, when i am trying to link with dynamic boost libs on mingw on win7(yes, official propose building on linux with cross mingw tool), there is error. because m4 will let test script to link with -lboost_chrono_<somethingofversion>.dll, but my mingw32 on win7 does NOT work. after fix, it works. i see that, m4 also try to support .dylib suffix, although, the proposed way is to link with static boost libraries. so can you confirm and verify this fix, thanks!