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.
Qt resource is innitialized by a static object constructor (see
https://wiki.qt.io/QtResources). When we put resources into a static
library, the linker removes that static objects and thus the resources
themselves. To correct that we append resources to the main executable
sources list. This is done via custom function qbt_target_sources which
knows where to read the executable' name.