MIPP_ENABLE_BACKTRACE=0 does not exclude `#include <execinfo.h>` which would be helpful for non-glibc builds

[marty-dy]

Setting MIPP_ENABLE_BACKTRACE to off does not exclude inclusion of execinfo.h which would be helpful in non-glibc builds.

[kouchy]

Thx, don't hesitate to send a PR!