Enable backtrace stuff only if GNU C library used, because current
backtrace implementation based on <execinfo.h> , which is not a
part of standard C library, it is a GNU extension.
This will be usefull when building with custom POSIX-compilant C
library (like musl) and no <execinfo.h> available.
Note: configure script will detect presence of <execinfo.h> and
enable/disable feature depending on it.
Feature is enabled by default.
