Refactor CMake build scripts

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.

dist/CMakeLists.txt

@@ -1,3 +1,5 @@
+find_package(Qt5Widgets ${requiredQtVersion}) # to conditionally install desktop-related files
+
 if (APPLE)
     add_subdirectory(mac)
 else (APPLE)