refactor: rename and re-organize CMake options (#401)

This improves usability of sdbus-c++ in downstream CMake projects. CMake options now have `SDBUSCPP_` prefix so potential conflicts with downstream options/variables are minimized. Also, all configurable options and variables are placed in a single place in the root CMake file.

docs/CMakeLists.txt

@@ -15,15 +15,19 @@ if(BUILD_DOXYGEN_DOC)
         # workaround bug https://github.com/doxygen/doxygen/pull/6787
         add_custom_command(TARGET doc POST_BUILD
           COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_LIST_DIR}/sdbus-c++-class-diagram.png ${CMAKE_CURRENT_BINARY_DIR}/html/.)
-
-        install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html DESTINATION ${CMAKE_INSTALL_DOCDIR} OPTIONAL COMPONENT doc)
     else()
         message(WARNING "Documentation enabled, but Doxygen cannot be found")
     endif()
 endif()
 
-install(FILES sdbus-c++-class-diagram.png
-              sdbus-c++-class-diagram.uml
-              systemd-dbus-config.md
-              using-sdbus-c++.md
-        DESTINATION ${CMAKE_INSTALL_DOCDIR} COMPONENT doc)
+if(SDBUSCPP_INSTALL)
+    install(FILES sdbus-c++-class-diagram.png
+                  sdbus-c++-class-diagram.uml
+                  systemd-dbus-config.md
+                  using-sdbus-c++.md
+            DESTINATION ${CMAKE_INSTALL_DOCDIR} COMPONENT doc)
+
+    if (BUILD_DOXYGEN_DOC AND DOXYGEN_FOUND)
+        install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html DESTINATION ${CMAKE_INSTALL_DOCDIR} OPTIONAL COMPONENT doc)
+    endif()
+endif()