Qt translations have to be compiled in a shared library or executable,
and since we use static libraries for the components, webui translation
files have to be compiled into the main executable.
1. Use FeatureSummary module to show configuration results.
2. Invert option()/find_package() relationship: instead of
calling find_package(... REQUIRED) when option is set, rely on optional
find package call and PackageName_FOUND variable.
3. Refactor handling options that result in simple preprocessor defines
(actually copy the snippet from libtorrent) so that everything is done
in a single function call.
4. Populate target properties in order to get rid of
include_directories() calls.
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.
