Update libunwind configuration check to look for unw_backtrace().
Update libunwind configuration check to look for unw_backtrace().

Update libunwind configuration check to look for unw_backtrace(), which
is a newer API not available in older versions of libunwind.

TeamExodus/external_jemalloc,AndroidExternal/jemalloc,sudosurootdev/external_jemalloc,sudosurootdev/external_jemalloc,AndroidExternal/jemalloc,VRToxin/android_external_jemalloc,CyanogenMod/android_external_jemalloc,TeamExodus/external_jemalloc,xin3liang/platform_external_jemalloc,CMRemix/android_external_jemalloc,geekboxzone/lollipop_external_jemalloc,geekboxzone/mmallow_external_jemalloc,android-ia/platform_external_jemalloc,geekboxzone/lollipop_external_jemalloc,android-ia/platform_external_jemalloc,CyanogenMod/android_external_jemalloc,geekboxzone/mmallow_external_jemalloc,sudosurootdev/external_jemalloc,TeamExodus/external_jemalloc,geekboxzone/mmallow_external_jemalloc,CMRemix/android_external_jemalloc,xin3liang/platform_external_jemalloc,AndroidExternal/jemalloc,AndroidExternal/jemalloc,CyanogenMod/android_external_jemalloc,CyanogenMod/android_external_jemalloc,xin3liang/platform_external_jemalloc,android-ia/platform_external_jemalloc,TeamExodus/external_jemalloc,geekboxzone/lollipop_external_jemalloc,android-ia/platform_external_jemalloc,sudosurootdev/external_jemalloc,geekboxzone/mmallow_external_jemalloc,geekboxzone/lollipop_external_jemalloc,VRToxin/android_external_jemalloc,CMRemix/android_external_jemalloc,VRToxin/android_external_jemalloc,VRToxin/android_external_jemalloc,xin3liang/platform_external_jemalloc,CMRemix/android_external_jemalloc