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.

.travis.yml

@@ -166,7 +166,10 @@ script:
     if [ "$build_system" = "cmake" ]; then
       mkdir build
       cd build
-      cmake -DGUI=${gui} -DCMAKE_INSTALL_PREFIX="$qbt_path" "$MY_CMAKE_OPENSSL_HINT" \
+      if [ "$gui" = "false" ]; then
+        DISABLE_GUI_OPTION="-DCMAKE_DISABLE_FIND_PACKAGE_Qt5Widgets=ON"
+      fi
+      cmake $DISABLE_GUI_OPTION -DCMAKE_INSTALL_PREFIX="$qbt_path" "$MY_CMAKE_OPENSSL_HINT" \
         -G "Ninja" -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=TRUE ..
       BUILD_TOOL="ninja"
     fi