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/lineedit/CMakeLists.txt

@@ -1,10 +1,13 @@
-set(QBT_LINEEDIT_SOURCES
+add_library(qbt_lineedit STATIC
 src/lineedit.cpp
-)
-
-set(QBT_LINEEDIT_HEADERS
 src/lineedit.h
 )
 
-add_library(qbt_lineedit STATIC ${QBT_LINEEDIT_SOURCES} ${QBT_LINEEDIT_HEADERS})
-target_link_libraries(qbt_lineedit Qt5::Widgets)
+target_link_libraries(qbt_lineedit
+    PRIVATE
+        qbt_gui_headers
+    PUBLIC
+        Qt5::Widgets
+)
+
+target_include_directories(qbt_lineedit PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src)