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.

src/gui/rss/CMakeLists.txt

@@ -1,26 +1,22 @@
-include_directories(${CMAKE_CURRENT_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR})
-
-set(QBT_RSS_HEADERS
+add_library(qbt_rss STATIC
 articlelistwidget.h
 automatedrssdownloader.h
 feedlistwidget.h
 htmlbrowser.h
 rsswidget.h
-)
-
-set(QBT_RSS_SOURCE
 articlelistwidget.cpp
 automatedrssdownloader.cpp
 feedlistwidget.cpp
 htmlbrowser.cpp
 rsswidget.cpp
-)
-
-set(QBT_RSS_FORMS
 automatedrssdownloader.ui
 rsswidget.ui
 )
 
-add_library(qbt_rss STATIC ${QBT_RSS_HEADERS} ${QBT_RSS_SOURCE} ${QBT_RSS_FORMS})
-target_link_libraries(qbt_rss qbt_base)
-target_link_libraries(qbt_rss Qt5::Gui Qt5::Widgets Qt5::Network)
+target_include_directories(qbt_rss PRIVATE ${CMAKE_CURRENT_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR})
+target_link_libraries(qbt_rss
+    PRIVATE
+        qbt_gui_headers
+    PUBLIC
+        qbt_base Qt5::Gui Qt5::Widgets Qt5::Network
+)